Final answer:
In the U.S., a person is considered an adult at 18 years old in terms of legal responsibilities. Option d is correct.
Step-by-step explanation:
In the United States, the concept of 'Once an Adult, always an Adult' is not a legal principle that determines the age of adulthood. In the U.S., a person is considered an adult at 18 years old in terms of legal responsibilities, such as voting, joining the armed forces, and taking on legal and financial responsibilities.
Therefore, the correct answer to the question is d) 50 states, as all states in the U.S. accept 18 as legal adulthood age.