(d) acceleration of the block.
Explanation:
Since the observers are moving with respect to each other at a speed v, they will find the block moving at different speeds. Kinetic energy observed by each one will be different at any time t. So the answer is not (a).
Since the direction of movement of the block is not given, each observer may see it moving a different distance. So they may observe different work done by friction.
Total work done on the block will be equal to change in K.E. But the velocities observed by both the observers are different. So for them change in K.E. will be different. Answer (c) is not correct.
Acceleration is the rate of change of velocity. Though for both observers velocities are different but the difference of velocities are same, means the rate of change of velocities (acceleration) is same. So the answer is (d).
