Meaning: 'Itself' is used to emphasize the subject, in this case, the stability of the noble gases, indicating that it is the main focus of the discussion.

Wrong options:

• himself: The word 'himself' is a reflexive pronoun used for males, which does not fit here as the subject is not a person.
• away: The word 'away' is an adverb indicating distance or removal, which does not relate to the context of stability or bonding.
• themselves: The word 'themselves' is a plural reflexive pronoun, which does not agree with the singular subject 'the stability'.

Correct answer: according

Explanation: The word 'according' is a preposition that introduces a phrase indicating the basis for Mendeleev's arrangement of elements. It fits grammatically as it correctly links the arrangement to atomic weight, providing a clear contextual relationship.

Meaning: 'According' means 'in agreement with' or 'based on,' indicating that the arrangement of elements was done based on their atomic weight.

Wrong options:

• contributing: The word 'contributing' is a gerund that implies an action of giving, which does not fit the context of how elements were arranged.
• need: The word 'need' is a noun or verb that does not provide the necessary relational context for the arrangement of elements.
• bond: The word 'bond' is a noun or verb that does not convey the intended meaning of arrangement based on atomic weight.

Correct answer: regularly

Explanation: The word 'regularly' is an adverb that describes the frequency of the appearance of noble gases in the periodic table. It fits grammatically as it modifies the verb 'appeared' and contextually indicates a consistent pattern in their occurrence.

Meaning: 'Regularly' means 'at regular intervals' or 'consistently,' indicating that noble gases appeared in a predictable manner in the periodic table.

Wrong options:

• enormously: The word 'enormously' is an adverb that indicates a large extent, which does not fit the context of frequency or pattern.
• abruptly: The word 'abruptly' suggests a sudden or unexpected occurrence, which contradicts the idea of a consistent pattern.
• vividly: The word 'vividly' refers to something being clear or striking, which does not relate to the frequency of appearance in the periodic table.