When can aircraft be prohibited from crossing the landing threshold for aircraft A behind A?

The correct choice highlights the fact that aircraft can be prohibited from crossing the landing threshold based on the ability to determine distances using suitable landmarks. This means that situational awareness regarding spacing and distance is crucial while managing aircraft on approach. When conditions allow for clear visual reference points, air traffic controllers can make informed decisions to safely sequence traffic and prevent potential conflicts, ensuring a smooth and safe landing process.

In contrast, prohibiting crossings at night (the first choice) may not be universally applicable since it depends more on the specific situational parameters rather than time alone. Requesting a crossing based solely on a pilot's request (the third choice) does not account for the objective safety standards that controllers must maintain regardless of a pilot's wishes. Finally, while heavy traffic conditions could influence decision-making, they do not directly address the importance of the ability to visually assess distances, which is the crux of the rationale behind the correct answer.