Final answer:
A child should be placed in a rear-facing child passenger restraint system if they are less than 1 year of age, weigh less than 20lbs, or are already in such a system; option D is correct.
Step-by-step explanation:
Under the conditions specified, a child should be placed in a rear-facing child passenger restraint system if all mentioned criteria are met: A) the child is less than 1 year of age, B) the child weighs less than 20lbs, and C) the child is already riding in a rear-facing child passenger restraint system. The option D) all of the above is the correct choice as it encompasses all the mentioned circumstances in which a rear-facing child passenger restraint system is appropriate for children.