Before opening the container valve, what must be securely attached?

The correct answer emphasizes the importance of having a regulator securely attached to the container valve before opening it. A regulator is crucial for controlling the flow of gas coming from the container, allowing for safe and manageable use. Without a regulator, opening the valve could release gas too quickly or at too high a pressure, which could lead to hazardous situations.

The regulator helps ensure that the gas is delivered at a safe pressure to the equipment being used, reducing the risk of accidents or equipment damage. It is standard safety protocol to have the regulator in place to maintain proper pressure levels and facilitate correct operation.

The other options, while related to gas handling, do not serve the same critical function as the regulator. For example, a pressure gauge would help in monitoring the pressure but does not control it. A cap is used primarily for safety when the container is not in use, while a safety valve provides a backup mechanism to prevent excessive pressure but is not involved in the immediate regulation of gas flow when opening the valve. Thus, ensuring the regulator is securely attached is a fundamental step in safely managing compressed gases.