The correct option is
C Unsociable
The word 'gregarious' refers to a person fond of company or sociable.
Option A, indecent refers to something not conforming to generally accepted standards of behaviour.
Option B, unsociable refers to someone not enjoying or making an effort to behave socially in the company of others.
Option C, above reproach refers to something perfect, for which no criticism can be made.
Option D, unlovable refers to something unlikeable or unfriendly.
Thus, according to the meaning, the word 'unsociable' is the most opposite in meaning from the word 'gregarious'.