Dextromethorphan is an effective over-the-counter (OTC) option for the treatment of which of the following conditions/symptoms?

A. Coronavirus: Dextromethorphan does not treat viral infections such as coronavirus. While it may help manage a symptom like a cough, it has no antiviral properties or effect on the underlying infection. B. Allergic rhinitis: Allergic rhinitis is primarily treated with antihistamines, intranasal corticosteroids, or decongestants. Dextromethorphan does not address nasal congestion, sneezing, or histamine-mediated symptoms. C. Nonproductive cough: Dextromethorphan is an antitussive that suppresses the cough reflex, making it effective for dry, nonproductive coughs where coughing provides no benefit and causes discomfort or sleep disturbance. D. Ineffective productive cough: In a productive cough, suppression with dextromethorphan is not recommended because coughing helps clear secretions from the airways. Using it in this situation could worsen airway obstruction or infection risk.