Meaning: 'Habit' refers to a regular practice or routine that is often done unconsciously.

Wrong options:

• when: 'When' is a conjunction that introduces a time clause, which does not fit as it does not describe a practice or routine.
• whenever: 'Whenever' is an adverb that indicates any time something happens, but it does not denote a practice or routine.
• until: 'Until' is a preposition that indicates a time limit, which does not describe a regular practice.
• bit: 'Bit' is a noun that refers to a small piece or amount, which does not relate to the concept of a routine or practice.
• yourself: 'Yourself' is a pronoun that refers back to the reader, but it does not fit as a noun describing a practice.
• less: 'Less' is an adverb that indicates a smaller amount, which does not relate to the concept of a routine.
• under: 'Under' is a preposition that indicates a position, which does not fit in the context of describing a practice.

Correct answer: whenever

Explanation: The word 'whenever' serves as an adverb that indicates any time an action occurs. It fits grammatically and contextually as it introduces the condition under which the reader should check their forehead.

Meaning: 'Whenever' means at any time that something happens.

Wrong options:

• when: 'When' is a conjunction that indicates a specific time, but 'whenever' is more appropriate here as it suggests any time without restriction.
• habit: 'Habit' is a noun that does not fit in this context as it does not indicate a time frame.
• until: 'Until' is a preposition that indicates a limit in time, which does not fit the context of checking at any time.
• bit: 'Bit' is a noun that does not relate to time and does not fit grammatically.
• yourself: 'Yourself' is a pronoun that does not indicate a time frame and is not appropriate here.
• less: 'Less' is an adverb that indicates a smaller amount, which does not relate to the timing of the action.
• under: 'Under' is a preposition that indicates a position, which does not fit in the context of time.

Correct answer: less

Explanation: The word 'less' is an adverb that indicates a smaller degree or amount. It fits grammatically and contextually as it describes the reduction in the frequency or intensity of headaches.

Meaning: 'Less' means a smaller amount or degree.

Wrong options:

• when: 'When' is a conjunction that does not indicate a degree or amount, making it unsuitable in this context.
• whenever: 'Whenever' is an adverb that indicates any time something happens, but it does not convey a reduction in amount.
• until: 'Until' is a preposition that indicates a limit in time, which does not fit the context of describing a reduction.
• bit: 'Bit' is a noun that refers to a small piece or amount, but it does not fit grammatically as an adverb describing reduction.
• yourself: 'Yourself' is a pronoun that does not relate to the concept of reduction and is not appropriate here.
• habit: 'Habit' is a noun that does not convey a sense of reduction in this context.
• under: 'Under' is a preposition that indicates a position, which does not fit in the context of describing a reduction.

Correct answer: yourself

Explanation: The word 'yourself' is a reflexive pronoun that refers back to the reader. It fits grammatically and contextually as it emphasizes the action of testing the therapeutic powers of smiling on one's own self.

Meaning: 'Yourself' refers to the person being addressed, emphasizing personal involvement.

Wrong options:

• when: 'When' is a conjunction that does not refer to a person and does not fit in this context.
• whenever: 'Whenever' is an adverb that indicates any time something happens, but it does not refer to a person.
• until: 'Until' is a preposition that indicates a limit in time, which does not fit as a reference to a person.
• bit: 'Bit' is a noun that does not relate to a person and is not appropriate here.
• habit: 'Habit' is a noun that does not refer to a person and does not fit in this context.
• less: 'Less' is an adverb that indicates a smaller amount, which does not relate to a person.
• under: 'Under' is a preposition that indicates a position, which does not fit in the context of referring to a person.

Correct answer: until

Explanation: The word 'until' is a preposition that indicates a time limit or condition. It fits grammatically and contextually as it describes the duration of the action of trying to smile until a change is noticed.

Meaning: 'Until' means up to the point in time that something happens.

Wrong options:

• when: 'When' is a conjunction that indicates a specific time, which does not convey the same sense of duration as 'until.'
• whenever: 'Whenever' is an adverb that indicates any time something happens, but it does not imply a duration or limit.
• less: 'Less' is an adverb that indicates a smaller amount, which does not fit in the context of duration.
• bit: 'Bit' is a noun that does not relate to time and does not fit grammatically.
• yourself: 'Yourself' is a pronoun that does not indicate a time frame and is not appropriate here.
• habit: 'Habit' is a noun that does not convey a sense of duration in this context.
• under: 'Under' is a preposition that indicates a position, which does not fit in the context of describing a duration.