Answer: The answer is D, trading.
Explanation: Growing stability in Europe in the late middle ages made possible extensive trade between East and West and within Europe itself. Italian city-states such as Venice and Genoa had trading ports in the eastern Mediterranean and the Black sea—trade that made these cities among the wealthiest cities in Europe. Most historians today generally agree that the plague was likely spread through Eurasia via these trade routes by parasites carried on the backs of rodents. The bacterium Yersinia pestis (and not all historians agree this was the culprit) likely traveled from China to the northwestern shores of the Caspian Sea, then part of the Mongol Empire and by the spring of 1346, Italian merchants in the Crimea, specifically the Genoese-dominated city of Kaffa (today Feodosiya in the Ukraine) brought the disease west. Rats carrying infected fleas boarded ships bound for Constantinople (today Istanbul in Turkey), capital of the Byzantine Empire. Inhabitants there were sickened by the plague by early July.
From these Greek-speaking lands, the plague spread to North Africa and the Middle East with terrible consequences; by autumn 1347, it had reached the French port of Marseilles and progressed both north and west. By early November, the Italian city-states of Genoa, Pisa, and Venice—commercial hubs for European trade—had been struck.
Most of the rest of Europe followed in short order. The disease spread along the active trade routes that northern Italian and Flemish merchants had developed. London and Bruges then communicated the disease via busy shipping lanes to the Nordic countries and the Baltic region (aided by a trading partnership known as the Hanseatic League). Western crusaders seeking to attack the Holy Land prompted innovations in shipbuilding and these larger and faster ships carried large quantities of goods over extensive trade networks— but they also carried the deadly pathogen.