hii

look here we now that the liquid which we substitute in place of the body is in equilibrium.

and the only force we know is the weight mg of the liquid downward....(1)

and so there must be a force  in the upward direction due to which it is in equilibrium

so this force we consider it to be the bouyant force say B which will act vertically upward in order to keep it in eq....

Hence              B =Mg ...(2)