Final answer:
A dashed blue line on a VFR sectional chart signifies Class D airspace around an airport with an operational control tower, active during specified tower operating hours.
Step-by-step explanation:
On a VFR sectional chart, a dashed blue line indicates Class D airspace. Class D airspace is typically depicted with a blue dashed circle around an airport that has an operational control tower. It is designed to facilitate a safe environment for aircraft operating under Visual Flight Rules (VFR) and is active only during the specified operating hours of the control tower. If the tower is not operating, the space becomes Class E or G airspace, where fewer restrictions apply, allowing for more freedom of movement for pilots.