Final answer:
A high school diploma is not required to become a naturalized U.S. citizen; other requirements include adulthood, permanent residency (green card), physical presence, good character, and English proficiency.
Step-by-step explanation:
The requirement that is not necessary for becoming a naturalized citizen of the United States is C. A high school diploma. The criteria for naturalization include being at least 18 years of age, being a lawful permanent resident (which means holding a green card), residing in the U.S. as a permanent resident for at least five years, being physically present in the U.S. for at least 30 months, being of good moral character and being able to speak, read, write, and understand the English language. There is no educational requirement such as having a high school diploma for the naturalization process.