Answer
D. The acceleration vector of the planet points towards the center of the circle.
Explanation
For planetary motion , the velocity vector is always in a direction that is tangential to the direction of motion, not towards the center. Even though the speed of the planet may stay constant, it's velocity changes because it is constantly changing it's direction.
The centripetal force, which in this case is the force of gravity always points in the direction of the the center. The acceleration vector always points in the direction of the of the center of the circle just like the centripetal force.
