Final answer:
The Portuguese presence in southern China was most directly enabled by technological developments in cartography and navigation (B).
Step-by-step explanation:
These technological advances allowed Portuguese explorers to travel farther and more safely across the oceans, effectively paving the way for establishing trade relationships with Asia, including China. The Portuguese and later the Spanish leveraged these developments to bypass traditional trade routes controlled by Muslim and Jewish merchants and to discover new sources of wealth, such as silver from the Americas, which played a crucial role in trading with China, where silver was in high demand as the base of their monetary system.
The influx of American silver facilitated trade because the Chinese economy had moved to silver as their monetary base. Despite this, the Portuguese role in trade remained relatively small given the vast size of the Chinese market, and their presence was bolstered by the demand in China for silver, which the European powers could supply in exchange for highly coveted Chinese goods.
Moreover, the expansion of trade networks and obtaining of spices from the Spice Islands enhanced Portugal's wealth and influence in the global trade landscape.