Final answer:
b. The port cities of Venice, Amsterdam, Hamburg, London, and Marseilles were early front-runners in urban development in Western Europe because they served as centers of trade and capital and labor mobility.
Step-by-step explanation:
The best explanation for why cities such as Venice, Amsterdam, Hamburg, London, and Marseilles were early front-runners in urban development in Western Europe is option B: These port cities were centers of trade, serving as links in terms of capital and labor mobility.
These cities were strategically located on major waterways, which allowed for easy access to trade routes. Trade brought wealth and economic opportunities, attracting people from different areas and contributing to population growth. Cities became centers of commerce and trade, fostering urban development.