A _____________ is a device that splits one hose stream into two different hose streams.

A wye is a crucial device in firefighting operations that allows one hose stream to be split into two separate streams. This functionality is particularly beneficial when a single water source needs to provide flow to multiple fire suppression lines simultaneously, enhancing operational efficiency and flexibility during firefighting efforts.

In practical applications, the wye is attached to a hose line and features two outlets, which enable firefighters to deploy additional lines without the need for a second water source or additional pumping equipment. This allows for a more streamlined approach to managing water flow and can be vital in situations where rapid deployment is necessary.

The other devices listed serve different purposes. A branch line typically refers to a section of hose in a layout that extends from a main line to deliver water to a specific point. A reducing valve is used to regulate water pressure, ensuring that it does not exceed safe working levels for hoses and equipment. Couplings are connectors that join different lengths or sections of hose, enabling easy assembly of hose lines but do not serve to split a stream. Thus, the wye stands out as the correct answer for the function of splitting one hose stream into two.