Which of the following is considered one type of marine engine?

A marine engine refers to any engine designed specifically for use on boats and other watercraft. An inboard engine is a common type of marine engine that is installed within the hull of the boat. This configuration allows for better space utilization and can contribute to a more stable ride, as the weight of the engine is lower and centralized. Inboard engines often provide efficient power and are typically used in larger vessels, where the engine's placement aids in overall balance and performance.

While options like electric engines and gasoline engines are also types of marine engines, they are more specific categorizations under the broad spectrum of inboard or outboard engines. Electric engines refer to those powered by electric batteries, while gasoline engines are those that run on gasoline, but both can be found in inboard configurations. The term "inboard engine" more directly defines a category based on its installation and function, making it an appropriate answer in this context.