What action is required if two aircraft are simultaneously arriving at the same fix at different altitudes?

Answer: B — Provide separation

When two aircraft are simultaneously approaching the same fix at different altitudes, providing separation is essential to ensure safety and prevent collisions. This action involves maintaining a safe vertical or lateral distance between the two aircraft, particularly since they are on a converging path at a common location. Air traffic controllers must assess the situation and ensure that the aircraft do not infringe upon each other's flight levels, as having them at different altitudes does not guarantee safety without implemented separation measures. By providing separation, the controller actively manages the traffic to maintain safety and smooth operations within the airspace, taking into account the performance characteristics and current traffic conditions. Maintaining altitude for both aircraft would not address the potential risk of confliction about the respective flight paths. Monitoring the situation alone lacks active intervention and does not offer a solution if a conflict arises. Redirecting one aircraft could be a separate measure but is not necessary if separation can be ensured at the same fix. Thus, providing separation is the most appropriate course of action.