What do approach lights mark for pilots?

Approach lights serve a crucial function for pilots as they indicate the threshold area identifying the runway. These lights are positioned along the extended centerline of the runway and come into view as aircraft approach for landing. By guiding pilots visually, approach lights help ensure that they are aligned with the runway for a safe landing. This helps in improving visibility during low-light conditions or adverse weather situations when visual references may be limited.

While the other options touch on aspects of flight safety and operations, none define the specific purpose of approach lights as clearly as identifying the runway threshold area does.