Which of the following is NOT one of the requirements for naturalization?

The correct answer is C, which states that one must "speak fluent Spanish." For naturalization in the United States, there is no requirement to speak any specific language, including Spanish. Instead, applicants must demonstrate an ability to read, write, and speak basic English, which is a requirement shared among all applicants, regardless of their native language. This ensures that naturalized citizens can engage effectively in civic duties and participate in society.

The other options represent established requirements for naturalization. Being at least 18 years old is necessary as it aligns with the requirement that applicants must be adults. Having lived in the U.S. for 5 years reflects the residency requirement, showing that applicants have established ties to the country. Lastly, passing the citizenship test is essential to demonstrate knowledge of U.S. history and government. Therefore, option C stands out as not being part of the naturalization requirements, as it specifies fluency in a particular language rather than a general capability in English.