Final answer:
All major colonial cities in British North America were seaports involved in the mercantile system under British colonial rule.
Step-by-step explanation:
The assertion that all major colonial cities in British North America were seaports is the most accurate characterization of these urban centers. By the 18th century, the coastal cities had become crucial hubs for the import of British goods and export of raw materials from the colonies. Cities like Boston, New York, Philadelphia, and Charleston were all bustling seaports that played a significant role in the mercantile system established under British imperial policies.
Although there was unrest and a growing sense of economic dissatisfaction leading up to the American Revolution, it would be an overstatement to say that all major cities were rife with working-class protests. Moreover, while smaller than European counterparts, colonial cities were still important trade centers and not inherently hostile to British mercantile power, as many colonists benefitted from trade. In sum, the most accurate statement is that major colonial cities were seaports playing a key role in the mercantilist economic system.