Correct option is A
The correct antonym of the given word is (a)
Mature.
Young: being in the early stage of life; youthful (Hindi: युवा)
Example: He is too young to vote.
Mature: fully developed in body or mind; adult. (Hindi: परिपक्व)
Example: He is mature enough to handle responsibility.
Synonyms of Young: juvenile, youthful.
Antonyms of Young: mature, aged, old.
Other options:
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(b) Boyish – still young/childlike (not opposite).
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(c) Stiff – rigid, unrelated.
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(d) Moist – slightly wet; unrelated to age.