Convertible seats when used rear facing should be used for which of the following?

The correct choice is comprehensive, as it encompasses all the important safety guidelines for using convertible car seats in a rear-facing position. When using convertible seats in this manner, these criteria ensure that infants are properly secured for maximum safety during travel.

Infants less than one year old and weighing up to about 20 pounds are particularly vulnerable due to their size and developmental stage. Therefore, using a rear-facing seat during this time period is essential, as this position supports the infant's head, neck, and spine, minimizing the risk of injury in the event of a crash.

Additionally, having the harness straps positioned at or below the infant's shoulders is a crucial safety feature. This positioning helps to hold the infant securely and prevents excessive movement within the seat, which could lead to injury.

Furthermore, setting the harness clips at the armpit level provides an additional layer of security. When properly adjusted, the clips keep the harness snug against the child's body and ensure that the straps fit securely without being too loose or too tight.

By meeting all these requirements, the use of convertible seats in a rear-facing position can significantly enhance the safety of infants while traveling, making the integrated approach outlined in this choice the most prudent.