The correct option is
C Warranted
The word 'gratuitous' refers to an action that is uncalled for and is done without good reason.
Option A, charitable refers to being generous in giving those who need.
Option B, grand appearance indicates something that is large and impressive.
Option C, warranted refers to the act of justifying a course of action.
Option D, being grateful is an act of feeling or showing an appreciation for something done or received.
Thus, according to the meaning, the word 'warranted' is the most different in meaning from the word 'gratuitous'.