In standard form, an ellipse's major axis is indicated by the terms like this:
In the top equation, the vertical axis is primary and in the second the horizontal axis is primary. That's a bit more info than the question asked, but I thought it may be helpful to understand the answer.
Now, a co-vertex is the intersection point between an ellipse and its minor axis. On the graph of the ellipse, the is the distance from the center to where the ellipse intersects its minor axis, so our answer is A.
If a graphical representation would be helpful, I would take a look at the Math Warehouse article on the Equation of an Ellipse in Standard Form.
the answer is d