A. When a conservative force does positive work on a body, the potential energy of the body increases/decreases/remains unaltered.

Explanation: When a conservative force does positive work on a body, the object is displaced in the direction of force and towards the centre of the force such that the distance between the source and the object decreases. So, potential energy also decreases.

B. Work done by a body against friction always results in a loss of its kinetic/potential energy.

Explanation: Friction acts always in the direction opposite to that of motion and results in a decrease in the velocity of the body. Hence, friction decreases the kinetic energy of the body.

C. The rate of change of total momentum of a many-particle system is proportional to the external force/sum of the internal forces on the system.

Explanation: Internal forces cannot change the total energy of a system in order to maintain a constant energy of the system. Hence, only external forces can bring about a change in the total momentum of a many-particle system.

D. In an inelastic collision of two bodies, the quantities which do not change after the collision are the total kinetic energy/total linear momentum/total energy of the system of two bodies.

Explanation: The total momentum of a system remains constant in a collision irrespective of the nature of the collision according to law of conservation of linear momentum.