What is a requirement for renting to a customer under 21 years of age?

The requirement for renting to a customer under 21 years of age includes the stipulation that the rental must be a replacement rental. This means that if a customer's personal vehicle is being repaired or is unavailable, they may rent a vehicle as a temporary substitute. This regulation is often in place because it provides a legitimate and necessary reason for those under 21 to rent a vehicle, rather than allowing unrestricted access to rental vehicles which could lead to higher risk situations.

Other options either represent general policies that do not specifically align with regulations pertaining to renters under 21 or present restrictions that may not be universally applicable. For example, while providing two forms of ID or having a driver over 25 may enhance their ability to rent, these are not specific requirements tied to a rental for someone under 21. The stipulation that they cannot rent any type of vehicles is also too broad, as the rental agreement for replacement rentals allows for exceptions under defined circumstances. Therefore, the requirement aligning with the concept of a replacement rental is what makes this option the correct answer.