How is the height of an obstruction indicated on a sectional chart?

The height of an obstruction is indicated on a sectional chart as "feet above ground and sea level." This measurement is critical for pilots as it provides a clear understanding of the height of the obstruction relative to both the ground elevation directly beneath it and mean sea level, which is the standard reference point for altitude measurements in aviation. Using both these references ensures that pilots can maintain safe altitudes when navigating near potential obstructions, thus enhancing safety during flight operations.

The options that reference ground or sea levels in isolation may not provide the full context needed for safe navigation, which is why indicating the obstruction height relative to both ground and sea levels is important in ATC practices.