Which came as a great surprise to my aunt wilma. a. sentence b. fragment

The correct option is a.  Sentence

Explanation:

A sentence has three main components (1) a subject, which correspond to the who in the sentence, (2) a predicate, that corresponds to the verb or action, and (3) a complete thought that make sense by itself.

In contrast,  a fragment corresponds to an incomplete sentence where it is lacked either the subject or the verb, or even both.  

Bearing in mind the described above, it could be said that the example provided; "Which came as a great surprise to my aunt Wilma." correspond to a sentence since in it is possible to notice the three main components of a sentence.

it is the background information on the characters and setting explained at the beginning of the story. the exposition will often have information about events that happened before the story began. the exposition is often the very first part of the plot.