If an additional resistance is placed in series with a DC series motor that is running at its rated speed without any additional resistance, the speed of the motor will decrease.

DC series motors have a speed-torque characteristic that shows a decrease in speed as the load torque increases. When the motor is running at its rated speed without any additional resistance, it means that the load torque is balanced by the motor’s torque output.

By adding an additional resistance in series with the motor, the total resistance in the circuit increases. This results in an increased voltage drop across the total resistance, including the added resistance. As a result, the voltage available for the motor decreases, leading to a reduction in the motor’s torque.

With a decrease in torque, the load torque exceeds the motor’s torque output, causing the motor to slow down. The decrease in speed helps restore the balance between the motor’s torque and the load torque.

Therefore, the correct answer is A: Decreases. When an additional resistance is placed in series with a DC series motor running at its rated speed, the speed of the motor decreases as the load torque exceeds the reduced torque output of the motor.