Explanation: A series DC motor connects its field winding in series with the armature. This configuration dictates its unique operating characteristics: * **A) Maximum torque at low speed:** At low speeds, especially during start-up, the back electromotive force (EMF) is minimal. This allows a very high current to flow through both the armature and the series field winding. Since torque is proportional to the product of armature current and field flux (which is also high due to high field current), series motors produce exceptionally high starting torque. * **B) Quick starting:** The high starting torque characteristic (explained in A) allows a series DC motor to accelerate rapidly from a standstill, enabling quick starting even under heavy loads. * **C) Poor speed regulation under varying loads:** As the mechanical load on a series motor decreases, the armature current (and thus the field current and flux) also decreases. With weaker field flux, the motor must spin much faster to generate the necessary back EMF to balance the applied voltage. Under no-load conditions, the motor speed can become dangerously high (runaway speed), making it unsuitable for applications where the load can be completely removed. Conversely, heavy loads cause a significant drop in speed. Since all three statements accurately describe the characteristics of a series DC motor, the correct answer is D.