Final answer:
Primary herpetic gingivostomatitis, caused by the herpes simplex virus type 1 (HSV-1), is the contagious condition listed. It results in mouth ulcers and is spread through direct oral contact.
Step-by-step explanation:
The condition that is contagious from the options provided is primary herpetic gingivostomatitis.
Primary herpetic gingivostomatitis is an infection caused by the herpes simplex virus type 1 (HSV-1). This virus is highly contagious, often spreading through direct oral contact. The HSV-1 infection can result in ulcers on the mucous membranes inside the mouth and is usually self-limiting, though it may require treatment such as antiviral medications in more severe cases.
The other conditions mentioned are not viral and are not typically spread through casual contact. Pemphigus is an autoimmune disease affecting the skin and mucous membranes, recurrent aphthous stomatitis has no infectious cause and is characterized by the appearance of canker sores in the mouth, and necrotizing ulcerative gingivitis is a severe form of gum disease caused by certain bacteria, which generally requires direct exposure to the bacteria, often seen in individuals with weakened immune systems.