Which of the following is NOT a source used to develop hydraulic pressure?

The correct answer is based on the understanding of hydraulic systems and the common sources used to generate hydraulic pressure in aircraft. Hydraulic pressure is typically developed using fluid pumps designed specifically for that purpose.

Hand pumps, engine-driven pumps, and variable-displacement pumps are all integral components of hydraulic systems in aviation. Hand pumps can manually create pressure for smaller systems, engine-driven pumps utilize the aircraft engines to provide power for hydraulic functions, and variable-displacement pumps adjust the flow of hydraulic fluid depending on system demands.

In contrast, water-powered pumps do not align with traditional hydraulic systems, as hydraulic systems typically use specialized fluids, often oil-based, designed to maximize efficiency and prevent freezing or other issues that water might cause. Thus, water-powered pumps are not a standard source for developing hydraulic pressure in aviation applications, making this choice the correct answer to the question.