Monroe Doctrine and Roosevelt Corollary--the Monroe Doctrine first set the policy that the western hemisphere was closed to colonization. The Roosevelt Corollary took it a step further by providing military behind the policy.
Following the independence movement in the US and throughout Latin America, the US wanted to protect the area from future colonization. The Monroe Doctrine made the statement and then 80 years later the Roosevelt Corollary enforced the doctrine with our navy and army force.
It all depends on the definition of the term Modern Era. The word modern comes from Latin word modo, which means right now or just now. It was used for the first time during the Renaissance in Italy to bring attention to the great rediscoveries of sciences, the arts, history and politics of Classical antiquity and the subsequent discoveries and progresses accomplished like the Age of Discovery.
If such definition is used as the basis for this question then the answer is definitely C.The establishment of global empires.
Indeed, since during the previous periods, Empires were limited to their immediate geographic areas. There had been some attempts to explore areas that were located much farther like the Vikings and their travels to Greenland and North America.
However, it was the Europeans: and to some lesser extent the Chinese, that actually discovered (in the literal sense of removing the cover) the Americas for the entirety of the world. The discovery paved the way for the emergence of Global Empires that were completely unprecedented in the history of humanity since they spanned several continents. For instance, the Global Spanish Empire Under Philip II of Spain in the 16th Century that spanned the continents of Europe, the Americas, Africa and Asia.
