Explanation: A DC series motor has its field winding connected in series with the armature winding. This unique configuration means that the current flowing through the field winding is directly proportional to the current flowing through the armature, which in turn is determined by the mechanical load on the motor. When the load on a DC series motor decreases, the current drawn by the motor decreases. A reduced current through the series field winding weakens the magnetic field. To generate the necessary back-electromotive force (back-EMF) to oppose the applied voltage with a weaker field, the motor must compensate by significantly increasing its rotational speed. Conversely, a heavy load leads to high current, a strong field, and slower operation. Therefore, its speed is inherently and strongly determined by the load. Options B (number of pairs of poles) and C (commutator) describe essential design components. The number of poles influences the motor's design speed and torque characteristics, while the commutator enables continuous unidirectional rotation. However, neither directly *determines* the motor's operating speed in response to dynamic load changes in the way the series field winding interacts with the load.