Torque developed by a D.C. motor depends upon

The correct answer is D. All above factors.

The torque developed by a DC motor depends on the following factors:

• Magnetic field: The strength of the magnetic field determines the amount of force that is exerted on the conductor.
• Active length of the conductor: The longer the conductor, the greater the force that is exerted.
• Current flow through the conductors: The greater the current, the greater the force that is exerted.

The torque developed by a DC motor can be calculated using the following formula:

$T = B \cdot I \cdot L$

where:

• $T$ is the torque in Newton-meters (N m)
• $B$ is the magnetic field strength in Tesla (T)
• $I$ is the current in amperes (A)
• $L$ is the active length of the conductor in meters (m)

The torque developed by a DC motor is proportional to the magnetic field strength, the current flow, and the active length of the conductor.