Which of the following is the most effective way to change the output of a wind-driven pump?

Increasing the fan blade pitch is the most effective method for changing the output of a wind-driven pump. When you adjust the pitch of the fan blades to a higher angle, you enhance the ability of the blades to catch and utilize the wind's energy. This allows the pump to generate a greater flow rate and improve its efficiency.

The reason this is particularly effective lies in the fundamental mechanics of how wind-driven pumps operate; they rely on the angle of the blades to optimize wind capture. A steeper blade pitch increases resistance to the wind, causing the fan to harness more power from stronger wind flows, thus amplifying the pump's overall output.

Other options, such as decreasing the fan blade pitch or reducing the speed of the motor, would either decrease the efficiency of wind utilization or limit the power available to the system. Changing the fluid type, while it might affect variables such as viscosity and flow resistance, does not directly correlate to adjustments in wind energy conversion capabilities, making it less effective in modifying pump output compared to adjusting the fan blade pitch.