Function of Governor:

• To minimize fluctuations in the mean speed which may occur due to load variation, the governor is used. 
• The function of the governor is to increase the supply of working fluid going to the prime mover when the load on the prime mover increases and to decrease the supply of working fluid going to the prime mover when the load decreases so as to keep the speed of the prime-mover almost constant at different loads.

Function of gear and belt:

• To transmit motion from one shaft to another.

Function of gyroscope:

• It is used to maintain stability in orientation and navigation.

Hence, the correct option is (C).