Which type of valve must be readily accessible and does not require tools for operating on a container for liquid or vapor?

The correct choice is the type of valve that ensures quick access in case of an emergency or routine operation. Main shutoff valves are designed to provide immediate control over the flow of liquid or vapor from a container. Their accessibility without the need for tools is essential for safety, allowing operators to swiftly shut off gas flow if necessary. This can be critical in preventing accidents or managing leaks, contributing to safe handling practices.

In the context of the other options, emergency relief valves are meant to activate automatically under specific pressure conditions but are not intended for manual operation. Pressure regulating valves function to maintain specific pressure levels within the system but again do not need to be accessed frequently. Relief pressure valves are designed to vent excess pressure but are also not the first line of control like main shutoff valves. Therefore, the main shutoff valve stands out as the one that must be readily accessible and tool-free for both safety and operational efficiency.