Which safety device is activated by the compression and extension of a landing gear strut?

The safety device activated by the compression and extension of a landing gear strut is the Ground Safety switch. This switch plays a crucial role in ensuring the safety of ground operations. When the landing gear strut compresses during landing or extension during takeoff, it activates the Ground Safety switch, which helps prevent accidental retraction of the landing gear while the aircraft is on the ground. This ensures that the landing gear remains safely deployed when it is crucial for ground operations, thereby preventing potentially dangerous situations.

The other options involve different functions: the Brake pressure switch monitors the hydraulic pressure in the braking system and is unrelated to the landing gear strut; the Anti-skid valve controls the braking system to prevent wheel skidding during landings, which also does not act upon the physical movement of the landing gear strut; and the Landing gear actuator is responsible for changing the position of the landing gear itself and does not serve the same safety role as the Ground Safety switch.