The correct answer is A. Subject
Step-by-step explanation:
In most sentences, the subject is one of the first elements; this always precedes the main verb, and can be identified because it is the person, entity or object the sentence is about or the entity acting. For example in the sentence "My sister jumps the rope" the subject is "my sister".
In the sentence presented, "Clover" which is the underlined word, acts as a subject because this precedes the main verb "is", and it is the entity that is being described through the verb and the subject complement (Jackie's beloved horse). Thus, the underlined word is used as a subject in the sentence.