Fill in the blank in the following sentence by choosing the correct option out of the ones that follow each sentence. 
I would have helped you if you __________ me. 

Correct option is C

The correct answer is (c) had asked.

The sentence is an example of a third conditional sentence, which is used to express an unreal or hypothetical situation in the past—something that did not happen, but we're imagining what the result would have been if it had.

The structure of a third conditional sentence is:
If + past perfect, would have + past participle

So, in this case:
"If you had asked me, I would have helped you."
is the correct and grammatically accurate form.