Answer :
C. This warm-up exercise is useful for actors, singers, and public speakers.
A. you can't put a colon after in , you just can put a colon after of and some other words.
B. you can't also put a colon after with
C. good
D. you need to put a colon after of
So I believe that C is correct
B. you can't also put a colon after with
C. good
D. you need to put a colon after of
So I believe that C is correct