What type of valve must be fitted to the vapor manifold?

The correct choice pertains specifically to the requirements for valves in the vapor manifold on tank vessels. A manually operated isolation valve is essential because it serves the critical function of allowing operators to control the flow of vapor and isolate the system when necessary. This capability is vital for ensuring safety and preventing unintentional releases during operations such as loading and discharging cargo where vapor management is crucial.

Using a manually operated valve enables the crew to respond quickly and effectively to any situation that might arise, such as a need to stop the flow of vapor in an emergency. Moreover, isolation valves are standard features that comply with regulatory requirements for safely managing the vapor phase while minimizing risk during cargo operations.

The other choices might have their merits in different contexts, but they do not fulfill the specific role required at the vapor manifold as effectively as a manually operated isolation valve does. Automatic or remote-controlled valves can introduce complexities and potential points of failure that might not allow for immediate manual intervention. Pressure relief valves, while critical for safety in other domains, are more focused on preventing over-pressurization rather than controlling the isolation of vapor flow directly.