What could non-citizens obtain to enter the United States?

A non-citizen can obtain a visa to enter the United States, which serves as an official permission for temporary stay. Visas can be granted for various purposes, including tourism, business, and study, and they typically outline the terms and duration of the stay. This legal document signifies that the individual has met specific requirements set by U.S. immigration authorities and is authorized to enter the country for a designated period.

A passport, while important for international travel, is primarily issued by a person's home country and does not grant access to another country. A green card provides permanent residency in the U.S. but is not applicable for initial entry. A work permit allows non-citizens to work legally in the U.S. but also does not serve as a means of entry itself. Therefore, the visa is the correct answer as it specifically allows non-citizens to enter the United States.