The correct answer is: B. Where load changes frequently.
D.C. series motors are used in applications where the load changes frequently, such as in cranes and hoists. This is because the speed of a D.C. series motor is proportional to the current, and the current is proportional to the load. This means that the speed of a D.C. series motor will change in response to changes in the load.
A. Where load is constant is not the correct answer because D.C. series motors are not well-suited for applications where the load is constant. This is because the speed of a D.C. series motor will decrease as the load increases. This can be a problem in applications where the speed needs to be kept constant, such as in conveyor belts.
C. Where constant operating speed is needed is not the correct answer because D.C. series motors are not well-suited for applications where the speed needs to be kept constant. This is because the speed of a D.C. series motor will decrease as the load increases.
D. In none of the above situations is not the correct answer because D.C. series motors are used in applications where the load changes frequently.