The word "Reading" in the phrase "Reading quickly and well requires practice" functions as a/an ___________.

Explanation

Option A refers to the base form of a verb preceded by 'to'. Option B is a verb showing action. Option C, a gerund, is a verb form ending in -ing that acts as a noun. Option D is a verb form used as an adjective or to form continuous tenses. In this sentence, "Reading" functions as a noun, so it is a gerund.