The correct option is D 4 m
Draw the free body diagram of block and platform. As M >> m, the acceleration of the platform, a=μmgM can be taken as zero and hence its velocity remains constant as 4m/s. Acceleration of the block is,
→ablock,platform=2m/s2 (towards right)
→vblock,platform=(2t−4)m/s (towards right)
Initial velocity of block wrt platform is 4 m/s towards left.
Let s be the required distance, then v2=u2+2→a.→s⇒0=(−4)2+2(2)s⇒s=4m.