I was shocked at his crass behaviour.

Answer: Option C

Indifferent : having no particular interest or sympathy; unconcerned.
Rigid : unable to bend or be forced out of shape; not flexible.
Stupid : having or showing a great lack of intelligence or common sense.
Difficult : needing much effort or skill to accomplish, deal with, or understand.
Crass : showing no intelligence or sensitivity.

Synonym of Crass is Stupid