Final answer:
A healthy adolescent will most frequently present with gingivitis during orthodontic treatment.
Step-by-step explanation:
During orthodontic treatment, a healthy adolescent will most frequently present with gingivitis. Gingivitis is the first stage of periodontal disease where microbial infection causes the gums to become inflamed and irritated with occasional bleeding. As for the other conditions listed, horizontal bone loss and vertical bone loss are generally more advanced stages of periodontal disease and necrotizing ulcerative gingivitis, also known as trench mouth, while concerning, is less common in healthy individuals, particularly those with access to good medical and dental care. Orthodontic treatment can exacerbate gingivitis if oral hygiene is not maintained, as braces and other apparatus can harbor food particles and bacteria.