Flywheel:

• A flywheel is used to control the variations in speed during each cycle of an engine.
• It maintains constant speed, but the means is different from that of Governor.
• It stores excess of rotational energy from the power stroke and supply back during non- power strokes of the cycle.

Governor:

• It controls mean speed of the engine under full/half/varying load conditions by regulating the supply of working fluid to the engine.
• Governor maintains constant mean speed over a period of time.

KEY POINT:

• A flywheel is not required in all prime mover engines, but Governor is required in all the prime movers (engines).

Hence, the correct option is (B).