What distinguishes a stern drive engine from an inboard engine?

A stern drive engine is defined by its combination of an inboard engine and an outdrive unit, which is typically mounted outside the boat. This setup allows the engine itself to be housed within the vessel, while the outdrive, which includes the propeller, is protruding from the stern (back) of the boat. The outdrive is connected to the inboard engine, allowing for improved performance and maneuverability compared to traditional inboard setups.

In this case, the correct answer highlights that the outdrive is specifically attached to an engine that is located inside the boat. This differentiates stern drives from purely inboard engines, which do not have an external propulsion unit but instead rely solely on internal mechanisms for power output.

The other options relate to various characteristics that do not accurately define the significant distinction between stern drive and inboard engines. For instance, the claim that the engine is completely outside the boat misrepresents how stern drives work. Additionally, the notion that the engine is not adjustable and the idea that the outdrive is used exclusively for jet propulsion do not align with the foundational concepts of how these propulsion systems operate.