__________ is responsible for wear and tear of machine parts.
(A) Oil
(B) Steel
(C) Friction
(D) Smoothness

Hint: Machine parts get worn out due to rubbing with each other over prolonged periods of time and as such lubricants are used to keep the machine in functioning conditions

Complete step by step solution:
Whenever we work with machines, after some time, they start to show signs of failure in the form of wear and tear. The prolonged use of the machines depends on how well we maintain it. As such the worn-out sections of the machine are replaced at periodic intervals under the banner of preventive maintenance. As such we can understand that wear and tear is not good for the machine.
One of the factors responsible for wear and tear in parts made of only iron has been rusting which has been a problem for ages. As such carbon and other elements were mixed into iron to prevent rusting, thus forming steel. So that in itself shows that steel prevents wear and tear to some degree which could have occurred, were simple iron parts used. As such option 2 is incorrect.
Coming back to the subject of wear and tear, replacing parts in the machine is costly and as such lubrication is done to prevent the wear and tear. One of the lubricants is oil and mineral oils are used in industries on a large scale. Now, oil has a property that smoothens things and is slippery in nature which is what is needed of a lubricant. As such option 1 and 4 are both incorrect.
This leaves option 3 friction as our answer. About friction, you know that it has a property of obstructing motion and as such, the machine will have to exert more force to employ motion in the part if there is less slipperiness on the surface of the part and so this will cause the machine to face wear and tear. Also, the fact that friction, which arrests motion also causes the surfaces in contact to slowly grind against each other, removing material causing further wear and tear.

Note:
When you get mobile oil for the motorcycles or scooters, it is nothing but a lubricant which goes to the engine and keeps the internal parts (ring and piston) in well lubricated condition, preventing wear and tear therein.