When must all men register for the Selective Service?

The correct answer is C. at age 18.

In the United States, all men are required to register for the Selective Service System within 30 days of their 18th birthday. This ensures that the government has a list of individuals who could be drafted in case of a national emergency. It’s important for eligible individuals to comply with this requirement to avoid penalties. If you have more questions or need further clarification, feel free to ask!