Class D airspace is depicted on a sectional chart with:
AA dashed blue line surrounding the airport, with a blue number in a square bracket showing the airspace ceiling in hundreds of feet MSL✓
BA solid magenta line and a 'D-' label
CA hatched green border
Why →Class D airspace on sectionals uses a dashed blue line surrounding the airport. The ceiling altitude is shown in a blue bracketed number (e.g., '[25]' means 2,500 feet MSL). Class D extends from the surface to the bracketed ceiling and typically has a 4-nautical-mile lateral radius.
The trap →The dashed blue can be confused with a Class C ring style, but Class C is solid magenta in current charts. Line style and color together identify the class.
Field note →Class D ceilings on the sectional are in hundreds of feet MSL, not AGL. A '[25]' ceiling at a field with 1,000 feet MSL elevation means 1,500 feet AGL of Class D.
SOURCE → FAA Aeronautical Chart User's Guide; AIM 3-2-5CHECKED JUL 16ACS II.A.K1EASY