Answer:
(C)

Explanation:
A line will be parallel to the y-axis if It’s x never changes.
In A, x is always being -y, so that’s going to change all the time.
In B, x is always becoming y, so that is changing too.
In C, x is always being 4, so this never changes.
In D, x is always 4 times y, so this changes.
Hope this helped!