Answer:
Towards the center of circle, which changes the direction of motion but not speed
Step-by-step explanation:
The force acting on an object when it is moving in circular path is called centripetal force. The formula for centripetal force is given by :

Where
m = mass
v = velocity
r = radius of circle
In a circular path, the velocity of object changes continuously while its speed remains constant. The centripetal force acts towards the center of circle. Hence, the correct option is (a) " towards the center of circle, which changes the direction of motion but not speed ".