Numbers such as '55' shown above a horizontal line next to airport information on a sectional chart typically indicate:
AThe field elevation (in feet MSL) of the airport✓
BThe CTAF frequency (in MHz, abbreviated)
CThe longest runway length (in hundreds of feet)
Why →In the standard airport data block on sectional charts, the number immediately following the airport name is often the field elevation in feet MSL. Runway length is shown as a separate number, typically with an 'L' suffix indicating lighting status. CTAF frequency is shown with a preceding symbol.
Field note →For drone pilots, field elevation matters for density altitude calculations and for understanding the approach path altitude of nearby manned traffic.
SOURCE → FAA Aeronautical Chart User's GuideCHECKED JUL 16ACS II.A.K5EASY