It seems likely that you are supposed to give Ellis Island as the answer to this question. This was the first stop in the United States for something like 12 million immigrants in the years 1892 to 1954 (with most of those immigrants coming between 1892 and 1924). However, Ellis Island was not, by any means, the first stop for all immigrants who came to the United States. There were huge numbers of immigrants who came to the US before 1892 and after 1924. There were immigrants who entered through the West Coast. None of these people had Ellis Island as their first stop in the US.
However, if Ellis Island is not the correct answer, it is hard to know what will be. Most immigrants to the US had large cities as their first homes. The large cities were home to more jobs that immigrants could take. They were also home to immigrant communities that could help to support new immigrants both emotionally and possibly even financially. If Ellis Island is not the answer, “large cities” would be my best guess for the correct answer.
Because Ellis Island is so famous, it is somewhat likely that it is the answer your teacher expects to see. However, you should be aware that it was only the first stop for a relatively small percentage of all the immigrants who have come to the United States over the years.