The correct option is B cheerful
The word 'sulky' refers to refusing to be cooperative and cheerful.
Option A, mild refers to being not severe, serious or harsh.
Option B, cheerful refers to being noticeably happy and optimistic.
Option C, loving refers to feeling o showing love and care.
Option D, kind refers to having a friendly, generous or considerate nature.
Thus, according to the meaning, the word 'cheerful' is the opposite of the word 'sulky'.