Which sentence uses the active voice? my research paper was finished two days early. my research paper is being finished two days early. i finished my research paper two days early. description?

"I finished my research paper two days early."

A good way to determine active vs passive is to ask who performed the action. Did the subject perform the action, or was the action performed "by" the subject?

The subject performs the action in this sentence:

I = [subject]
finished = [action]
my research paper = [thing on which the action was performed]

In the other examples, the subject has an action performed on it. 

My research paper = [subject]
was finished/is being finished = [action]
two days early = [time frame information, not really important for the discussion here]

At this point, you have no indication as to who will finish the paper, do you? No. You assume that the paper will be completed "by me," but it could just as easily be completed "by my friend."

If you can add "by [person/thing]" to a sentence, it's passive voice. If you can't, it's not. Clearly, you would never say "I finished my research paper by me." 

Aan c sound like the correct ones.