The correct answer: C. Nickel.

Recognize the plaques on the tongue most consistent with oral lichen planus. Oral lichen planus has been associated with dental restorative materials, such as gold or nickel, which may be associated with oral lichen planus.

Balsam of Peru, formaldehyde, paraphenylenediamine, and potassium chromate are not as strongly associated with contact stomatitis as nickel in dental materials.

References:

Olejnik M, Jenerowicz D, Adamski Z, Czarnecka-Operacz M, Dorocka-Bobkowska B. The prevalence of contact hypersensitivity in patients with oral lichen planus. Postepy Dermatol Alergol. 2022;39(4):668-674.