An air to close valve is the same as which of the following valves?

An air to close valve operates by using air pressure to keep the valve open and will close when the air supply is removed. This functionality aligns with the characteristics of a spring to open valve. In this type of mechanism, when air pressure is applied, it overcomes the force of a spring that keeps the valve in the closed position, holding it open. If the air supply fails or is removed, the spring force then acts to close the valve.

The other options describe different types of valve behaviors or actuation methods that do not match the operation of an air to close valve. A fail-open valve, for example, operates in the opposite manner and would remain open when there is a failure in the control signal. A pneumatic valve generally refers to valves that use compressed air for actuation but does not specify the fail position. A manual valve is completely operated by human intervention and does not rely on air pressure at all. Thus, the spring to open designation precisely captures the functionality of an air to close valve, making it the correct answer.