I put D and got it right. I hope I helped!
Answer:

Explanation:
Given

Required
Determine the directrix
First, we express the equation in form:

Where the directrix is:

So, we have:

Rewrite as:

Multiply the right hand side by 4/4


By comparison:
and 



The directrix is calculated as:

So:


I think the answer would be A. after sentence 14! I hope I helped at least a little :)