What must a pilot be aware of when flying through an alert area?

When flying through an alert area, a pilot must be particularly aware of activities that are conducted in that space, as these areas are designated for various types of activities that may pose risks to aircraft operations. The activities in an alert area are typically marked on aviation charts with a prefix "A," which serves as an important indicator for pilots.

This awareness is critical, as alert areas can include things like training activities, parachuting, glider towing, or other operations that may not be visible or predictable to the pilot. Therefore, while there may not be any hard restrictions on flying through these areas, a pilot must stay vigilant and be prepared to avoid any potential conflicts with the activities taking place.

Maintaining altitude alone, as suggested in one of the other options, does not ensure safety without the corresponding situational awareness of what activities are occurring around the aircraft. Hence, being alert for activities marked with the prefix "A" on charts is the correct approach for ensuring safe operations in these zones.