Final answer:
Atopic dermatitis is a common skin disorder in the United States, with an estimated prevalence of 40 million Americans.
Step-by-step explanation:
The correct answer is D) 40 million. According to the information provided, acne is the most common skin disorder in the United States, and at any given time, at least 40 million Americans have acne. While the question specifically asks about Americans with atopic dermatitis, and not acne, it is important to note that both are common skin disorders. Therefore, it is reasonable to expect a similar prevalence for atopic dermatitis in the United States.