The correct answer is A. Speed.
Iron losses are the losses that occur in the magnetic circuit of a machine. They are caused by the eddy currents and hysteresis losses in the iron core. The eddy currents are caused by the changing magnetic field, and the hysteresis losses are caused by the repeated magnetization and demagnetization of the iron core.
The eddy current losses are proportional to the square of the current, and the hysteresis losses are proportional to the frequency of the current. Therefore, the iron losses are proportional to the square of the current and the frequency of the current.
The speed of a DC machine does not affect the current or the frequency of the current. Therefore, the iron losses are independent of the speed of a DC machine.
The load on a DC machine does affect the current. Therefore, the iron losses are dependent on the load on a DC machine.
The voltage on a DC machine does not affect the current or the frequency of the current. Therefore, the iron losses are independent of the voltage on a DC machine.