Which type of smoke detector operates mains powered and has a battery backup?

The type of smoke detector that operates on mains power while also having a battery backup is indeed the multiple station detector. These detectors are designed to work in conjunction with others in a system, allowing for interconnected alarms that can alert occupants throughout a building when smoke is detected. The mains power ensures they have a consistent source of energy, while the battery backup provides reliability in case of power outages, maintaining their functionality when it is most critical.

In contrast, while ionization and photoelectric detectors can also function both as mains-powered and include backup battery options, the term "multiple station detector" specifically refers to a system of interconnected alarms that operate this way. Combination detectors typically refer to devices that incorporate multiple sensing technologies (like ionization and photoelectric) into one unit, but they do not specifically define the operational power mode associated with multiple station setups. Thus, the answer accurately depicts the type of smoke detection technology specified in the question.