The correct answer is (B) a fragment. A sentence typically requires an explicit subject and a predicate to express a complete thought. While "Listen and practice" contains two verbs in the imperative mood, implying an understood subject "you" (e.g., You listen and you practice), it lacks an explicitly stated subject. In formal grammar, a clause without an explicit subject, even if it conveys a complete command, is often classified as a fragment, especially when not part of a larger discourse.
Distractor (A) "a sentence" is incorrect because, despite conveying a command, its structure is not a complete independent clause with an explicit subject. While imperative sentences often omit the subject, in a strict grammatical classification context, they are sometimes considered fragments or a special type of sentence.
Distractor (C) "a phrase" is also incorrect. A phrase is a group of words without a subject-verb combination that acts as a single part of speech (e.g., "in the park," "running quickly"). "Listen and practice" contains two verbs, making it more than just a phrase; it's a clause (albeit an incomplete one without an explicit subject). It functions as a command, but grammatically, it's considered a fragment due to the omitted subject.