What should you do if you see another aircraft on a collision course at the same altitude?

The primary action when two aircraft are on a collision course is to determine the appropriate evasive maneuver based on their relative positions. In the case of two aircraft approaching each other head-on, the standard practice is to turn to the right. This is rooted in the concept of "right of way" in aviation, where both pilots are instructed to alter their course to the right to avoid a collision.

Option D, which suggests that both aircraft should give way to the right, is correct because it follows this established protocol for deconfliction in the air. This maneuver minimizes confusion and provides a clear response for both pilots, as turning right is a universally accepted practice in aviation for head-on encounters.

The other options do not adhere to the correct guidelines set forth in aviation regulations and protocols for aerial navigation and collision avoidance. Ascending or descending indiscriminately does not clarify the intended actions by either pilot and could result in an even greater risk of collision. Likewise, turning to the left is not advisable in this scenario, as it contradicts the standard right-turn practice when encountering another aircraft head-on.