The sentence that uses a infinitive phrase as a modifier is option C. The best way to cook a grilled cheese is with a strip of bacon in the middle.
An infinitive phrase is the infinitive form of a verb plus any complements and modifiers. The infinitive phrase can be a subject, an adjective, an adverb, a direct object, or a predicate nominative in a sentence. In the case of and adjective it tends to modify a noun placed before.
