Final answer:
The rule for a 21-year-old licensed driver accompanying a student driver is to provide guidance and assistance.
Step-by-step explanation:
According to the National Minimum Drinking Age Act of 1984, a licensed driver who accompanies a student driver must be at least 21 years old. This rule is implemented to ensure the presence of a mature and experienced individual in the vehicle, who can provide guidance and assistance to the student driver. The 21-year-old licensed driver is expected to be responsible and follow all traffic laws, as they are ultimately responsible for the actions of the student driver.