The correct option is C of acing the exam
A noun clause is a dependent clause that functions as a noun. It usually
begins with what, whatever, whom, that and other such relative
pronouns. A noun clause can act as a subject, object, predicate
nominative or object of a preposition, in a sentence.
We can reduce a noun clause to a noun phrase by replacing the verb with a gerund, like in option B, where the verb 'ace' is converted to a gerund 'acing'. Hence option C is correct.
Option A is incorrect because it is still a clause that has changed in meaning to the negative.
Option B is incorrect because it is still a clause with the relative pronoun 'that' removed.
Option D is incorrect because the answer lies in C.