The correct answer is C) increase in speed. In a diesel-electric plant, raising the generator's field excitation current will increase the generator's output voltage. This higher voltage will then be applied to the DC propulsion motor, causing an increase in the motor's speed. The higher generator field excitation allows the generator to produce more electrical power, which then drives the DC propulsion motor at a higher speed. The other options are incorrect because: A) raising the generator field excitation does not just affect the generator speed, but the propulsion motor speed as well; B) the effect is an increase in speed, not a decrease; and D) the higher generator field excitation alone, without changes to the generator engine speed, is sufficient to increase the propulsion motor speed.