The correct option is B cooperative
The word 'acrimonious' refers to a speech or discussion in a angry or bitter tone.
Option A, informal refers to having a relaxed, friendly or unofficial manner or style.
Option B, sympathetic refers to feeling, showing or expressing sympathy.
Option C, cooperative refers to involving mutual assistance in working towards common goal.
Option D, charitable is relating to the assistance of those in need.
Thus, according to the meaning, the word 'cooperative' is the opposite of the word 'acrimonious'.