The centripetal force exerted on stunt motorcyclist Biker Bob while riding on the inner vertical surface of a circular track is

a. friction.

b. his weight. - incorrect

c. the normal force.

d. none of the above

As we can see the figure

When a stunt motorcyclist is performing the stunt then as per force diagram we will have a its force of gravity or weight acting downwards along the surface of cylinder

The normal force due to vertical wall is towards the center of circle

and friction force is counter balancing the weight of motor cyclist

Now here we can see that the Normal force is towards the center of the circle

So correct answer will be

c. the normal force.