Any idea why c. is the right answer?

*Answer credited to @Asafmen*

This is a classic **Harmonic Circular Movement**.

To help it make more sense, let’s look at the vectors from above:

If you leave the ball to be free at any moment, it will continue to move in a straight line- not in a circle. If the acceleration was in the same direction as the speed, the movement would not be circular.

Using Newton’s Second Law: **F=ma**, we know **a** and **F** are vectors, but Mass **m** is scalar, proving **a** and **F** mostly have the same directions.

To simplify using an example, take a satellite rotation on the Earth, though not perfectly harmonic, the Earth in the middle is the centre.