Two observers are moving relative to one another at a speed v along...

Since the two observers are moving relative to each other at a constant speed $v$, they are in different inertial frames of reference.

According to Galilean relativity, the acceleration of an object is invariant across all inertial frames. If $u$ and $u'$ are the velocities of the block measured by the two observers, they are related by $u' = u - v$. Differentiating with respect to time $t$, we get $\dfrac{du'}{dt} = \dfrac{du}{dt}$ since $v$ is constant. Thus, $a' = a$.

The displacement of the block depends on the frame of reference. Consequently, the work done by friction ($W = \int F \cdot dx$) and the total work done will be different for the two observers.

Similarly, the kinetic energy ($K = \dfrac{1}{2}mu^2$) depends on the observer's frame since velocity is a relative quantity.

Therefore, only the acceleration of the block will be identical as measured by both observers.