What is the proper condition for accurately checking the brake wear indicator pins during preflight?

The correct choice indicates that the parking brake should be set and the hydraulic system pressurized to accurately check the brake wear indicator pins during preflight. This condition ensures that the aircraft's braking system is in an optimal state for a thorough inspection.

When the parking brake is set, it holds the aircraft stationary, which is crucial for checking components such as the brake wear indicators. The pressurization of the hydraulic system is also essential because it simulates the conditions under which the brakes would normally operate. This allows for a realistic assessment of whether the brake wear indicator pins are functioning correctly, as they provide feedback based on hydraulic pressure and movement.

Properly simulating operational conditions helps ensure that any wear or malfunctions in the braking system can be accurately identified before flight, contributing to overall safety. Other options, such as having the aircraft in flight or with the hydraulic system unpressurized, do not create the necessary conditions for inspecting the brakes effectively.