Which of the following are types of aircraft hydraulic systems?

The correct choice highlights the two specific types of hydraulic systems used in aircraft: conventional and fly-by-wire systems. Conventional hydraulic systems rely on hydraulic fluid under pressure to operate various control surfaces and components in the aircraft. These systems have been the standard for many years and utilize mechanical components to generate and manage hydraulic pressure.

Fly-by-wire systems represent a modern advancement where traditional mechanical linkages are replaced by electronic controls. In fly-by-wire systems, pilot inputs are transmitted electronically, and the aircraft's flight control computer interprets these signals, controlling the hydraulic actuators as necessary. This system offers advantages such as weight savings, increased reliability, and the ability to incorporate advanced control laws for improved handling characteristics.

In contrast, the other options mention other unrelated systems or combinations that do not accurately reflect the types of hydraulic systems typically found in aircraft. For example, mechanical and solar systems are not recognized as hydraulic systems in aviation, while hydro-pneumatic elements are more about the combination of hydraulics and pneumatics but do not specifically categorize hydraulic systems in the same way that conventional and fly-by-wire do.