Answer:
(d) (3a, a)
Explanation:
You want to know the upper right vertex of a rectangle with long-side coordinates (0, 0) and (3a, 0) such that the side in the x-direction is 3 times the side in the y-direction.
Width
If the length is 3a and is 3 times the width, then the width is 'a'.
Vertex
The point at upper right will have its x-coordinate equal to 3a, and its y-coordinate equal to 'a'. That point is (3a, a).