What do blue numbers on a chart signify?

The correct answer is that blue numbers on a chart signify GPS Minimum Enroute Altitude (MEA). This altitude is critical for ensuring safe navigation in the airspace, especially when flying under instrument flight rules (IFR). The GPS MEA provides a minimum safe altitude that pilots should maintain to ensure they are adequately above terrain and obstructions while navigating along a defined route in an instrument environment.

These altitudes take into account the curvature of the Earth and the potential hazards that could be encountered. By following the GPS MEA indicated by the blue numbers, pilots can avoid terrain and obstacles while also ensuring reliable GPS navigation.

In the context of the other choices, ground clearance levels are generally not indicated by numbers on charts, and visual flight rules altitudes are usually depicted differently, primarily relevant under VFR conditions. Instrument approach altitudes are also indicated on charts, but they are typically associated with specific approaches rather than general navigation routes. Therefore, understanding the significance of blue numbers is essential for maintaining safe altitudes during IFR operations.