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

Puerto Rico is considered a U.S. territory because it is an unincorporated territory of the United States, which means it is governed by U.S. federal law but does not have the same status as a state. This status allows Puerto Rico to have its own local government and constitution while still being under U.S. sovereignty. Residents of Puerto Rico are U.S. citizens, but they do not have voting representation in Congress and cannot vote in presidential elections, highlighting the unique status of U.S. territories compared to states.

The other choices, California, New York, and Pennsylvania, are all states within the United States, providing full state rights and representation in Congress. This distinction is important as it reflects the different governance and representation issues associated with U.S. territories versus states.