since the density =mass /volume if volume is constant we can rewrite the density equation as volume=mass / density since volume is just a constant c*density=mass where c is some constant that denotes the constant volume mass and density will be proportional to each other or in other words will be have exactly in the same way if one increase the other will increase if one decreases the other will decrease if one stays constant the other will stay constant only the fourth choice satisfies the proportionality condition so it must be the correct choice
