that was a good question actually. the answer is B because initially u have the mass m, the height h and the acceleration g. water flows to the second vessel until the height is equal, so the initial potential energy loses half the initial height AS WELL AS half the initial mass. g remains unchanged as it is always constant. so ( m/2 x h/2 x g ) gives u mgh/4
i hope u understood now
i hope u understood now