The water in the vessel is half down by gravity. To make it rise in the vessel, we need an additional force opposing the gravity. The pumping of air does not provide this force, hence option (A) is untrue.

As the air is pumped out, the number of air molecules decreases and hence the pressure decreases. Option(B) is therefore correct.

The force on the bottom of the vessel is due to the weight of the water and the pressure due to air. As it is pumped out, the total force on the bottom of the container decreases. This makes option (C) correct.

Changing air pressure does not affect the density of the liquid. Hence, option (D) is wrong.