The correct answer is A. indifferent.
Crass behavior is rude, offensive, or uncivilized. It is the opposite of polite or refined. Someone who is indifferent is not interested or concerned about something. They are not easily impressed or excited.
B. Rigid means inflexible or unyielding. It is the opposite of flexible or adaptable. Someone who is rigid may be difficult to work with because they are not open to new ideas or changes.
C. Stupid means lacking intelligence or common sense. It is the opposite of intelligent or wise. Someone who is stupid may make careless mistakes or say things that are offensive or hurtful.
D. Difficult means hard to deal with or manage. It is the opposite of easy or manageable. Someone who is difficult may be demanding or unreasonable.
In the context of the question, the speaker is shocked at the other person’s rude and offensive behavior. This suggests that the other person is indifferent to the speaker’s feelings.