What safety feature is installed on propane cylinders to allow excess pressure to escape?

The correct choice is the relief valve, which is specifically designed to allow excess pressure to escape from propane cylinders. Propane, being a flammable gas, can create dangerous situations if the pressure within the cylinder becomes too high due to temperature changes or overfilling.

The relief valve acts as a safety mechanism that automatically opens at a predetermined pressure level, venting gas to the atmosphere and thereby preventing potential rupture of the cylinder. This is crucial for maintaining safety during the storage and transportation of propane, as it mitigates the risk of explosion or fire caused by over-pressurization.

While a pressure gauge provides valuable information regarding the current pressure in the cylinder and a shut-off valve allows for controlling the flow of propane during usage, neither of these components serve the same safety function as the relief valve. The regulator also plays a role in controlling the pressure of propane as it exits the cylinder, but it does not provide a mechanism for relieving excess pressure dormant within the cylinder itself.