Choose the appropriate word from the options given below to complete…

The correct answer is Option (3) → rather

Adverb of degree before an adjective:

• Rather is an adverb of degree meaning fairly, to some extent; it comes before an adjective.
• This sentence: "Jatin is rather mature for his age" says he is fairly mature compared with others of his age.
• Complete sentence: Jatin is rather mature for his age.
  Example: The film was rather long but very enjoyable.

Why the Other Options are Incorrect?

Option 1: enough — Incorrect. As an adverb, enough comes after the adjective: "mature enough", not "enough mature".
Example: He is old enough to travel alone.

Option 2: much — Incorrect. Much goes with a comparative (much more mature), not with a plain adjective like "mature".
Example: She is much taller than her sister.

Option 4: least — Incorrect. Least is a superlative and needs "the" and a group to compare with (the least mature of all).
Example: He was the least worried person in the room.

Contrast: rather comes before the adjective (rather mature); enough comes after it (mature enough).