The correct option is
C Restrained
The word 'boisterous' refers to being noisy, energetic and cheerful.
Option A, friendly refers to being favourable or serviceable.
Option B, sincere refers to being free from pretence or deceit.
Option C, humble refers to having or showing a modest or low estimate of one's importance.
Option D, restrained refers to being characterized by reserve or moderation.
Thus, according to the meaning of the given word, the word 'restrained' is most different in meaning to the word 'boisterous'.