Here the observers are moving with respect to each other at a speed u, they will find the block moving at different speeds. Kinetic energy observed by each one will be different at any time t. Hence, option A doesn’t satisfy. Since the direction of movement of the block is not given, each observer may see it moving at a different distance. So, they may observe different work done by friction. Total work done on the block will be equal to change in its kinetic energy. But the velocities observed by both the observers are different. So, for them the change in kinetic energy will be different. Hence option C does not satisfy. Whereas acceleration is the rate of change of velocity. Even though for both the observer’s velocities are different, but the difference of velocities are same, means the rate of change of velocities is same. Hence only option D satisfies our answer.