The only forces on the particle are gravity and the normal force. The normal force acts in a direction orthogonal to the tangential direction. Therefore, it does not contribute to the acceleration. So, we need only find the component of the gravitational acceleration in the tangential direction. The gravitational acceleration is
The derivative of with respect to gives a tangent vector:
So, the UNIT tangent vector is
Therefore, the answer is
Therefore, answer (D) is correct.