Which of the following is considered a U.S. territory?

Puerto Rico is considered a U.S. territory because it is an unincorporated territory of the United States, meaning that it is under U.S. jurisdiction but does not have the full rights of a state. Residents of Puerto Rico are U.S. citizens, and they have a local government, but they do not have voting representation in Congress or the right to vote in presidential elections. This unique status has implications for both governance and the legal rights of its inhabitants.

The other choices represent either foreign nations or territories that are not part of the United States. The British Virgin Islands and Spain are independent or semi-autonomous territories under different governments, while Mexico is a sovereign nation, not affiliated with the U.S. in the same way that Puerto Rico is. Therefore, Puerto Rico stands out as the answer reflecting a U.S. territory.