What component of a keg admits gas to maintain pressure?

The component that admits gas to maintain pressure in a keg is the valve. This valve is typically located where the gas line connects to the keg and allows for the flow of gas, usually CO2, into the keg. By introducing gas into the keg, pressure is maintained, ensuring that the beer remains carbonated and can be pushed out through the tap when needed.

In this context, the coupler, spear, and headspace play different roles. The coupler is a device used to connect the keg to the beer system and often houses the valve but does not itself directly manage gas flow for pressure maintenance. The spear is the internal part of the keg that draws beer from the bottom and does not interact with the gas. Headspace refers to the space above the liquid inside the keg, which is important for allowing gas to expand but does not actively manage gas flow. Therefore, the valve is the correct answer as it specifically serves the critical function of admitting gas to maintain keg pressure.