Final answer:
Croup (option B) is the viral illness characterized by inspiratory and expiratory stridor and a seal-bark-like cough that is most common in children under the age of 3 years.
Step-by-step explanation:
The viral illness characterized by inspiratory and expiratory stridor and a seal-bark-like cough that is most common in children under the age of 3 years is Croup (option B). Croup is caused by viral infections, most commonly the parainfluenza virus. It leads to inflammation and swelling of the voice box (larynx) and windpipe (trachea), resulting in the distinctive symptoms of stridor and seal-bark-like cough. It is usually a self-limiting condition that improves with time and supportive treatment.