ANS:D - all (a), (b) & (c)

Lubrication is done to control all (a), (b) & (c): (a) Friction: Lubrication reduces friction between moving surfaces by providing a lubricating film that separates them. This reduces the resistance to motion and minimizes energy loss due to frictional forces. (b) Wear: Lubrication helps to reduce wear between contacting surfaces by providing a protective layer that prevents direct metal-to-metal contact. This extends the service life of machine components and reduces the rate of wear and tear. (c) Temperature: Lubrication also helps to control temperature by dissipating heat generated during operation. The lubricant absorbs and carries away heat from the frictional surfaces, preventing excessive temperature rise and maintaining the operating temperature within acceptable limits. Therefore, lubrication plays a crucial role in controlling friction, wear, and temperature, thereby improving the efficiency, reliability, and longevity of mechanical systems.