The correct answer is: A. To supply traction load.
A DC series generator is a type of DC generator that uses a series field winding. The series field winding is connected in series with the armature winding, which means that the current flowing through the field winding is the same as the current flowing through the armature winding. This results in a high starting torque, which makes DC series generators ideal for applications where a high starting torque is required, such as traction loads.
Traction loads are loads that require a high starting torque, such as electric locomotives and subway trains. DC series generators are often used to power traction loads because they can provide the high starting torque that is required.
Option B is incorrect because DC series generators are not typically used to supply industrial load at constant voltage. This is because the output voltage of a DC series generator is proportional to the field current, which means that the output voltage will vary with the load current. This is not desirable for applications where a constant voltage is required, such as industrial loads.
Option C is incorrect because the voltage at the toad end of the feeder is not typically determined by the type of generator that is used. The voltage at the toad end of the feeder is determined by the length of the feeder and the load current.
Option D is incorrect because DC series generators are used for a variety of purposes, including traction loads.