Hydraulic systems 2 and what other system power the slats?

The correct answer is that the slats are powered by hydraulic systems 2 and 3.

This is because the C-17 utilizes a redundancy system in its hydraulic setup to ensure that critical flight surfaces, such as the slats, have reliable power sources under various conditions. Hydraulic system 2 primarily serves various flight controls, while hydraulic system 3 supports additional needs, including slat operation. The combination of these two systems provides both efficiency and backup capability, ensuring that the slats can extend and retract as necessary for optimal aerodynamic performance during different phases of flight.

It is important to understand the role of each hydraulic system in the C-17. Hydraulic system 1, while crucial for other flight controls, does not specifically power the slats, and neither do systems 4 or any other alternatives in this context. Thus, the unique cooperation between systems 2 and 3 highlights the aircraft's design for redundancy and safety in critical control surfaces.