Option D is the correct option. "You must use it wisely". "Must" is a modal auxiliary verb. Modal auxiliary verbs are added to another verb to show necessity, possibility, or capability. Like other auxiliary verbs, modal auxiliary verbs are not the main verb, but they change their meaning slightly. They are parts of speech. There are eight parts of speech in English grammar- noun, pronoun, verb, adjective, adverb, preposition, conjunction, and interjection.