Final answer:
Children younger than 6 years old or less than 4 feet 9 inches tall must be in a child seat in the back. This applies in the case of D: 6 years, 4 feet 9 inches.
Step-by-step explanation:
In this case, the correct option is 6 years, 4 feet 9 inches. According to the question, children younger than 6 years old or less than 4 feet 9 inches tall must be in a child seat in the back. This means that children who are 6 years old or older or taller than 4 feet 9 inches can sit without a child seat in the back.
This guideline is based on safety standards that are designed to protect children in the event of a car accident. Children under the age of 8 or shorter than 4 feet 9 inches generally do not fit properly in adult seatbelts, and the use of a car seat or booster helps position them for maximum safety. It's important to note that these rules can vary by state, so it's always best to check the specific laws in your area.