Keep in mind that this type of question will always result in a highly-opinionated answer, and that the question may stem from a highly-influenced leftist view.
In essence, the teacher wants you to see the act through the lenses of him being a despicable, bloody European who ruined the Americas. However, to answer the question:
Christopher Columbus is not the "true discoverer" of America, as there has already been people living in the Americas, thereby forfeiting "discovery". However, Columbus is not without the right to be called a discoverer. He, after all, opened up the America's and the information surrounding the "western Asia" to the Europeans of the day, effectively "discovering it".
How can we explain the dilemma we have on us? First, we must identify key words and define them to be able to answer the question.
Firstly, the word discoverer. As defined by the Oxford Language Dictionary, a "discoverer" is, quote: the first person to find or explore a place. (end quote). the argument the prompt would like to suggest is that Columbus is not the true discoverer of the America, as he was neither the first person in the world sense, nor was he the first person in a European sense, to discover the "New World". However, the word itself should be put in a subjective term. Take for example, the "discovery of steel, under the Sir Basil Brooke. In putting it into context of the common beliefs of Democrats, the examples would place it so that he was not the discoverer of steel, as the Chinese have been creating steel from the 3rd AD. However, it does not change the fact that it is still commonly believed that Brooke was the "discoverer" of the modern-steel, and that is the fact. The same is for Columbus. While it may not be that he is the first person to discover the Americas, he is the "true" discoverer in that he opened up the existence of the America's extensively to the Europeans, which led to exploration and settlements.
The next word we need to define is "true". True in the sense of this context can be best defined by dictionary.com, which states that truth is "being in accordance with the actual state or conditions; conforming to reality or fact; not false." It is true that Columbus is not the first discoverer, but once again, we should fall back to what I would like to call "collection of information age". The discovery of one place can occur "multiple times" with different people groups, and though one may think it is not discovered, the appearance of the unknown is always the discovery of something to a group. Christopher Columbus may not be the first discoverer, but he is a (note "a") true discoverer of America in a sense, as he does find a new unknown land, wrote about it, reported his finding to the Spanish crown, and help spearhead the Spanish's fuel to ' " discover " ' this new land. Truly, he is not the first european to find the Americas, neither is he the first human to set foot on the lands of the west, however, it is safe to say that Columbus has the right to say that he has explored the unknown lands of the west, and brought about it's existence to public knowledge of Europe. In the sense of European point-of-view, he has truly discovered the Americas.
To conclude, it entirely depends on whose viewpoint you are looking through. Both sides can be righted or wrong based on the subjective viewpoint, but one thing will not change, Columbus as well as many other Europeans before and after him helped introduce the existence of the lands westward to the people of Europe, and that it helped bring the world closer, albeit slowly and with small steps. Whether or not Columbus was the true discoverer is a small detail that entails a person who shifts to become "subjectively right" in this increasingly snowflake truth, and who cannot take history of how it is supposed to be, a lesson that entails to those who fail to learn from it, to be repeated, albeit under different circumstances. We must understand that we should not dwell on the miniscular facts of "who discovered what", but learn from the outcomes of the discovery, in which Europeans would later move in, and destroy the ways of life of the Native Americans, and bring about a large shift in how world history would continue.
