The torque developed in a DC series motor is directly proportional to the armature current (Ia).
In a DC series motor, the magnetic field produced by the field winding is in series with the armature winding. As the armature current increases, the magnetic field strength also increases. This stronger magnetic field interacts with the field produced by the series field winding, resulting in an increased torque.
Therefore, the correct answer is D. The torque developed in the motor is proportional to the armature current (Ia).