Aphthous stomatitis can be described as:

Aphthous stomatitis, also known as canker sores, is characterized by the presence of painful, shallow ulcers that can appear in various locations within the mouth, including the inner lips, cheeks, and tongue. The correct description mentions that this condition can occur as either a single ulcer or a cluster of ulcers. This variability in presentation is a hallmark of aphthous stomatitis, allowing it to manifest in different patterns, which can vary among individuals.

This condition is not contagious, thus distinguishing it from common viral infections that would be classified as option A. While some individuals may experience other symptoms or complications, aphthous stomatitis is not typically associated with fever, ruling out option C. Additionally, it is not restricted to just the gums; it can occur in soft tissue areas of the mouth, making option D incorrect.

Understanding these characteristics helps clarify why the presence of either single or clustered ulcers is a defining feature of aphthous stomatitis.