Final answer:
The lowest altitude an aircraft can legally operate between specific fixes on an airway is determined by the Minimum Enroute Altitude, which is indicated on aeronautical charts. A definitive answer requires access to the chart for airway T257.
Step-by-step explanation:
To determine the lowest altitude at which an aircraft can operate on a segment of an airway, one must refer to the aeronautical charts for that specific airway. These charts provide Minimum Enroute Altitudes (MEAs), which are the lowest published altitude between radio fixes that ensures acceptable navigational signal coverage and meets obstacle clearance requirements. Without the chart for airway T257, it is impossible to give a definitive answer. However, the MEA is usually indicated on the chart with each segment of the airway, showing the lowest safe altitude for that segment. The altitude options provided (4000, 5000, 6000, 7000 feet) suggest discrete choices likely representing MEA values for different segments of T257 or perhaps Minimum Obstacle Clearance Altitude (MOCA) within a certain distance from a VOR (typically within 22 nautical miles).