Which of the following are examples of indirect characterization of a main character? select all that apply.
a: what the main character says
b: the thoughts of the main character
c: a description of how the main character looks
d: how another character reacts to the main character

Indirect characterization is the act of showing instead of telling.
All four would be examples of indirect characterization of a main character, because the writer isn't telling us, or giving his/her opinion on the character, he is painting a picture for the reader, showing instead of telling. 

Telling a reader how a character dresses or what his inner thoughts are, shows the reader what the character is like, as opposed to saying. He seemed afraid you would say, 'he worried about what was going to happen.'

All four would be great examples of indirect characterization