The correct answer is A) increase in speed. In a diesel-electric plant, the generator's field excitation current controls the strength of the magnetic field in the generator. Increasing the field excitation current will increase the generator's output voltage, which in turn will increase the voltage supplied to the DC propulsion motor. This higher voltage will cause the DC motor to spin faster, resulting in an increase in the propulsion motor's speed. The other options are incorrect because: B) changing the generator field excitation does not directly affect the generator speed, C) increasing the generator engine speed in conjunction with the field excitation would be required to impact the main motor speed, and D) increasing the field excitation will result in an increase, not a decrease, in the propulsion motor's speed.