The correct answer is: B. Torque is proportional to armature current.
DC motors are preferred for traction applications because they have a high starting torque and can be easily controlled. The torque of a DC motor is proportional to the armature current, which means that the more current that flows through the armature, the more torque the motor will produce. This makes DC motors ideal for applications where a lot of starting torque is required, such as in traction applications.
The other options are incorrect because they do not accurately describe the relationship between torque and armature current in a DC motor. Option A states that torque and speed are inversely proportional to armature current. This is not correct, as torque and speed are not directly related to each other. Option C states that torque is proportional to the square root of armature current. This is also not correct, as torque is directly proportional to armature current. Option D states that the speed is inversely proportional to the torque and the torque is proportional to the square of armature current. This is also not correct, as speed is not directly related to torque and torque is not proportional to the square of armature current.