Answer:
a. at either A or B
Explanation:
Kinetic energy may be defined as the energy of the system or an object which is due to its velocity of the object it possess.
In the context, an object having mass is attached to spring which is vertical and the object moves up and down due to spring effect between points A and B. Now these points A and B are the extreme points after which the object bounces back.
At point A and B, the velocity of the object becomes zero and hence the kinetic energy of a body varies directly proportional to its velocity.
i.e. Kinetic energy