Where would you most likely find gate valves?

Gate valves are primarily used in water supply systems, including hydrants and sprinkler systems, to control the flow of water. Their design allows for either full flow or complete shut-off, making them ideal for these applications. In the context of fire protection, being able to quickly control water flow is critical during firefighting operations, which is why they're found in such settings. The installation of gate valves ensures that firefighters have immediate access to water supply while also allowing for quick shutdown when necessary.

The other choices present scenarios that do not commonly utilize gate valves. Portable fire extinguishers typically use other types of valves designed for quick activation rather than the bulky gate valve system. Hose couplings focus more on the connection between hoses rather than controlling flow. Fire trucks, while equipped with various types of valves and controls, usually do not have gate valves directly on the apparatus itself; instead, they may utilize other specialized systems to manage water flow from onboard tanks or external sources.