he does nothing but ___ the whole morning.

选c, watch TV
如果“but(除了)”前面有do的某种形式，则but后面用“动词原形”。如果but前面没有do的某种形式，则but后面用 “to + 动词原形”
如：
He had no choice but to go home.
He did nothing but play computer games all day.

选择a)