Forces acting on Block B are shown below,
Let us take weight of block B as W. This force of weight W acts on block B. Since the man exerts force (say F) above the level of block B there may be movement tendency between both blocks and a force of friction (P) will be applied by block A on block B downwards. There will be a force R acting upwards on the block B by the floor such that R=W+P. The push force F by man on block A which is transferred to block B through A is counterbalanced by a force of friction between block B and the floor. It is equal but opposite in direction to F.
