The correct option is C c
Let’s examine the sentence: “If you don't start behaving properly, I will be forced to tell to your supervisor.” The sentence uses the transitive verb “tell.” A transitive verb requires an object to complete its sense. Hence, the verb “tell” would be followed by the object “your supervisor” and not the preposition “to.” So, the preposition “to” should not exist between “tell” and “your supervisor.” Hence, the error is in Part c of the sentence. So, Option C is the correct answer.