What is one required condition for the preceding aircraft before allowing another aircraft to cross the landing threshold during the day?

The correct answer is that the preceding aircraft must have touched down before allowing another aircraft to cross the landing threshold during the day. This is crucial for ensuring safe operations at the airport.

When an aircraft is cleared to land, it must actually make contact with the runway to confirm that it has safely landed and is no longer in the landing phase. Allowing another aircraft to cross the landing threshold while the first aircraft is still airborne could lead to potential collisions or unsafe situations on the runway, as the second aircraft could interfere with the landing or turn-off of the first.

While other conditions mentioned, such as being on final approach or cleared for takeoff, are relevant in different contexts, they do not provide the assurance needed that the first aircraft has safely completed its landing. Being stabilized on approach simply indicates that an aircraft is correctly configured for landing, but it does not guarantee that it has landed yet. Touchdown is the definitive moment where the operational focus can shift to managing the subsequent landing traffic safely.