Meaning: 'Challenged' means to question or dispute something, particularly an established idea or belief.

Wrong options:

• pale: The word 'pale' is an adjective that describes color or brightness, which does not fit the context of disputing an idea.
• performed: The word 'performed' is a verb that implies carrying out an action, which does not relate to the context of questioning a scientific idea.
• prompted: The word 'prompted' is a verb that suggests causing something to happen, but it does not convey the idea of disputing or questioning an established belief.
• existed: The word 'existed' is the past tense of 'exist,' which does not fit grammatically or contextually in this sentence.
• distinct: The word 'distinct' is an adjective that describes something as being different or unique, which does not fit the context of challenging an idea.

Correct answer: existed

Explanation: The word 'existed' is the past tense of the verb 'exist,' which fits grammatically as it aligns with the past context of the sentence. It indicates that atoms were not the smallest particles that were present in the universe, which is a logical continuation of the thought.

Meaning: 'Existed' means to have been present or to have lived at a certain time.

Wrong options:

• pale: The word 'pale' does not fit grammatically or contextually, as it is an adjective and does not relate to the existence of particles.
• performed: The word 'performed' is a verb that implies carrying out an action, which does not fit the context of discussing the existence of particles.
• challenged: The word 'challenged' is a verb that does not fit in this context, as it does not relate to the existence of particles.
• distinct: The word 'distinct' is an adjective that describes differences, which does not fit the context of existence.

Correct answer: distinct

Explanation: The word 'distinct' is an adjective that describes characteristics that are clearly different or unique. It fits grammatically as it describes the noun 'characteristics' and contextually emphasizes the differences among the parts of atoms, which is relevant to the discussion of atomic models.

Meaning: 'Distinct' means clearly different or unique from something else.

Wrong options:

• pale: The word 'pale' is an adjective that describes color or brightness, which does not fit the context of describing characteristics.
• existed: The word 'existed' is a verb that does not fit grammatically or contextually, as it does not describe characteristics.
• performed: The word 'performed' is a verb that implies carrying out an action, which does not relate to the description of characteristics.