The correct answer is A. Fragment: contains no subject
Step-by-step explanation:
In grammar, a complete sentence is one that contains a subject (agent of the action), and a predicate that includes a verb (main action) and other words that create a complete idea. On the other hand, a fragment is a set of words that does not include all the elements necessary to have a complete sentence.
This occurs in the case of "Studied the lesson carefully" because in this set of words there is a predicate that includes the verb "studied" and the words "the lesson carefully" that explain the verb. However, the sentence does not say who studied and therefore the agent or subject is missing. Thus, this group of words is a fragment because it contains no subject.