Which boat engine type is known to be mounted through the transom?

The correct choice pertains to the sterndrive engine because it is designed to be mounted through the transom of a boat. Unlike inboard engines, which are typically housed entirely inside the boat, a sterndrive combines an inboard engine with an outboard drive system. The engine is mounted inside the boat, while the drive unit and propeller extend into the water through the transom. This design allows for greater maneuverability and the ability to tilt the drive unit out of the water for maintenance or storage.

In contrast, outboard engines are mounted externally on the transom and are entirely exposed above the waterline, making them easily removable. Inboard engines, as mentioned, are installed inside the boat’s hull, not through the transom at all. The term "propeller engine" is not standard terminology used to describe a specific type of engine; rather, it refers more generally to any engine that drives a propeller, which can include outboard, inboard, and sterndrive engines. Thus, the sterndrive engine is indeed the one type specifically recognized for its mounting location through the transom.