Final answer:
To become president of the U.S., one must be a natural-born citizen, above the age of thirty-five, and a resident in the U.S. for at least fourteen years.
Step-by-step explanation:
The formal constitutional requirement for becoming president of the United States is that one must be a natural-born citizen. Additionally, a candidate must be at least thirty-five years of age and must have been a resident in the United States for at least fourteen years. Therefore, the correct answer to the question is c. One must be a natural-born citizen.