Hi there!
I believe your answer is A. Asymptote. It wouldn't be focus or axis, those just don't make sense. This means it could be either asymptote or directrix. Directrix is a fixed line describing a curve or surface. An asymptote is a line that continually approaches a given curve but does not meet it at any finite surface. Therefore, your answer is asymptote.
Hope this helps!
- Chris
