The optimal maritime route linking the western United States (USA) to Western Europe is identified by the following sequence of ports:
- San Francisco (III): Positioned on the USA's west coast, this port serves as the initial nexus for numerous trans-European voyages.
- Honolulu (I): Residing in the Pacific, it functions as a critical intermediate point for global trade routes connecting the USA with both Asian and European markets.
- Colon (IV): Situated at the Caribbean gateway to the Panama Canal, Colon is an indispensable hub for maritime traffic between the USA and Europe.
- London (II): As the ultimate destination, its location in Western Europe finalizes the transcontinental link. Consequently, the correct selection is (D).