Underline the correct alternative :

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

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

(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.

(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.

(a)
A conservative force does a positive work on a body when it displaces the body in the direction of force. As a result, the body advances toward the centre of force. It decreases the separation between the two, thereby decreasing the potential energy of the body.

(b)
The work done against the direction of friction reduces the velocity of a body. Hence, there is a loss of kinetic energy of the body.
Work Energy Principle
$W=changeinKE$
since Work done by Friction is negative, it results in decrease of KE

(c)
Internal forces, irrespective of their direction, cannot produce any change in the total momentum of a body. Hence, the total momentum of a many- particle system is proportional to the external forces acting on the system.
since there is no external force acting
$F_{ext}=\frac{dP}{dt}$

(d)
The total linear momentum always remains conserved whether it is an elastic collision or an inelastic collision
since there is no external force acting
$F_{ext}=\frac{dP}{dt}$