Final answer:
The main seaport of Massachusetts was limited at the start of the American Revolution due to the siege of Boston, as local militias surrounded the city and restricted access.
Step-by-step explanation:
The event during the American Revolution that limited Massachusetts citizens' ability to use their main seaport at the start of the revolution was B. the siege of Boston. The siege began after the first engagements at Lexington and Concord when New England militias surrounded the city, effectively trapping the British forces inside.
As a result, Boston's seaport could not be used for regular trade or supply lines, constraining the movement and activities of both the British and the local citizenry.