Block On An Accelerating Surface

For the block to move with the truck, static friction between the block and floor must supply the force that accelerates the block at the same rate as the truck. The required force is $ma$, and friction can provide at most its limiting value $\mu mg$. The block stays put as long as $ma\le \mu mg$, so the maximum acceleration is when these are equal: $a_{max}=\mu g=0.4\times 10=4{\text{ m/s}}^2$. Notice the mass cancels, so the threshold is independent of how heavy the block is. The 2 m/s^2 value uses half the coefficient or omits a factor. The 6 m/s^2 and 8 m/s^2 values exceed the friction limit, meaning the block would actually slip backward relative to the truck at those accelerations. This is the NCERT and JEE friction-as-the-accelerating-agent scenario, and it overturns the common misconception that friction always opposes motion; here static friction actually points forward, in the direction of motion, because that is the only way to drag the block along with the truck. Were the truck to accelerate beyond $\mu g$, the block would lag and kinetic friction would take over at a fixed value, no longer able to keep pace. As a check, the answer $\mu g$ has units of acceleration, and a rougher floor with larger $\mu$ would let the truck accelerate harder before sliding, which is physically reasonable.