The correct answer is D) the high-pressure unit and then flows through a crossover to the low-pressure unit. In a cross-compounded turbine propulsion plant, steam enters the high-pressure turbine first, and then flows through a crossover pipe to the low-pressure turbine. This allows the steam to be used more efficiently, as the high-pressure steam can perform work in the high-pressure turbine before being directed to the low-pressure turbine for additional work extraction. The other options are incorrect because they do not accurately describe the flow of steam in a cross-compounded turbine system. Option A would not be possible, as the steam cannot enter all three turbine units simultaneously. Option B is incorrect because the steam does not flow directly to the condenser from the high-pressure unit. Option C is incorrect because the steam does not enter the high and low-pressure units simultaneously.