Final answer:
The reference material does not provide specific requirements for unsupervised driving under a Probationary License for those under 21. State-specific regulations define the eligibility for unsupervised driving, which may include age, driving hours, and completion of driver's education.
Step-by-step explanation:
The main answer to the question regarding the conditions under which a holder of a Probationary License under 21 years of age may drive unsupervised is not explicitly provided in the reference material. The provided information discusses the National Minimum Drinking Age Act of 1984 and other unrelated legal statutes. To meet the requirements for unsupervised driving, typically the individual must pass certain state-specific milestones, which could involve holding the probationary license for a minimum period, completing a required number of driving hours, or reaching a certain age.
It is important to consult your specific state's Department of Motor Vehicles (DMV) or equivalent agency for detailed information on the requirements for a Probationary License holder under 21 to drive unsupervised. These requirements vary by state and may include restrictions based on the time of day, the presence of adult passengers, or completion of driver education courses.