Final answer:
Gingivitis is the condition that only affects the gums, characterized by redness, swelling, and irritation, and may progress into periodontitis if left untreated.
Step-by-step explanation:
The student asked which condition only affects the gums. Of the options given, gingivitis is the correct answer. Gingivitis is the inflammation and irritation of the gums and can lead to more serious conditions like periodontitis if not treated. Periodontitis affects not only the gums but also the bone and ligaments supporting the teeth. Other conditions listed, such as periodontitis, xerostomia (dry mouth), dental caries, and candida, are not solely gum-specific as they either affect other parts of the mouth or are associated with broader oral health issues.