Select the most appropriate option to fill in the blank.
He is a smug and very ______ person who thinks only about himself.

Correct option is C

The correct option to fill in the blank is (c) egoistic.
"Egoistic" describes someone who thinks only about themselves.
Meanings of other options:
· Arsonist: a person who commits arson.
· Diffident: modest or shy because of a lack of self-confidence.
· Humble: having or showing a modest or low estimate of one's importance.