English phrase question (urgent plz)?

"Hearing" is right, and there is only one reason for it. "Hear" is a verb, but adding "-ing" in this case does NOT turn it into progressive tense. It turns a verb into a "gerundive," a verb form that functions as a NOUN. And why a noun? It has to be the object of the preposition "to," and only a noun will do for that function.