The correct option is B g4
If we look at the entire system as a whole, then the weight of the 5 kg block, which is 50 N, is the force that accelerates both the 5 kg block and the 15 kg block. In other words, the weight of one block is accelerating the equivalent of four 5 kg blocks, giving only one-fourth as much acceleration as the block would have if it were falling freely.