Answer
D is the vertex.
Work Step by Step
Ellipse are the set of all points in a plane for which the sum of the distance from two fixed points ${{F}_{1}}$ and ${{F}_{2}}$ is constant.
The vertices are x-intercepts points like $\left( -a,0 \right)\left( a,0 \right)$ and y-intercepts points like $\left( 0,b \right)\left( 0,-b \right)$of an ellipse.
Thus, D.
