Explain what makes a complete sentence.

A noun or pronoun and a verb are enough.
John runs.
He is reading. A bit of a cheat. 

To know if something is a sentence, there are three criteria:

1. It begins with a capital letter (e.g. F, D, or whatever letter needed).
2. It includes and end mark. This can be a period (.), question mark (?) or exclamation point (!).
3. It must contain at least one main clause, which contains an independent subject and verb. 

Answer might be b. i think