The correct option is D c
Let’s examine the sentence: “At the annual function of the school the principal advised to the students to be ideal citizens.” The sentence uses the transitive verb “advise.” A transitive verb requires an object to complete its sense. Hence, the verb “advised” would be followed by the object “the students” and not the preposition “to.” So, the preposition “to” should not exist between “advised” and “the students.” Hence, the error is in Part c of the sentence. So, Option C is the correct answer.