Friction is a/an
A. self-adjusting force
B. necessary evil
C. important force in daily life
D. all of the above.

Hint: Friction is a contact force that comes into play when the surfaces of two bodies (in contact) have some relative motion with respect to each other. Friction creates a lot of difficulties. However, it is useful in many cases.

Complete answer:
Friction is a contact force that comes into play when the surfaces of two bodies (in contact) have some relative motion with respect to each other. Therefore, frictional force is a type of opposing force. It opposes the relative motion between two bodies. It is due to friction that we find it difficult to rub to bodies or to slide two surfaces on each other.

Friction is a self-adjusting force. This means that it can adjust itself according to the relative motion between the two bodies. The type of friction that can adjust itself according to the force applied on the bodies is called static friction. Friction creates many disadvantages for humans. Since friction opposes the relative motion between two surfaces, it causes wear and tear of the surfaces.

Friction performs a negative work on a moving body and this work is converted into heat. In machineries, friction makes it difficult for the components to slide over each other. However, friction also provides many benefits in our daily lives. It is due to the friction between our feet and the ground we are able to walk. Objects like wheels roll due to friction. Therefore, friction is said to be a necessary evil and important force in our daily lives.

Hence, the correct option is D.

Note: Friction is contact force. This means that frictional force will come into play only when the surfaces are in contact. If two surfaces have relative motion with respect to each other but they are not in contact then, then there will be no friction acting on them .