A valve applied to operate another valve or control.

• A. Pilot Valve
• B. Live Load
• C. Twist
• D. Shuttle Valve

Answer: (A)

The term describes a valve that is used to control another valve—the pilot valve. In hydraulic and pneumatic systems, a pilot valve is a small valve that sends pressure to the actuator of a larger, main valve, thereby shifting its position to open, close, or modulate flow. This lets you control a big valve with a small, easily manipulated signal, often from a remote switch or sensor.

That’s why this is the best answer: the function defined—“a valve applied to operate another valve or control”—is exactly what a pilot valve does, acting as the control element for the main valve.

The other terms don’t fit because they refer to unrelated concepts: live load is about variable loads on structures, twist isn’t a standard valve term, and shuttle valve selects between two pressure sources rather than controlling a second valve.