Stains on the teeth may occur:

Stains on the teeth can occur in various forms and locations, and option D accurately reflects this by stating that they can exist within plaque and calculus or adhere directly to the tooth surface. This encompasses both extrinsic and intrinsic staining.

Extrinsic stains are those that occur on the external surface of the teeth, commonly caused by foods, beverages, and habits such as tobacco use. These stains often accumulate within deposits like plaque and calculus, which are dental biofilms and hardened deposits that can adhere to teeth.

On the other hand, intrinsic stains originate from within the tooth structure, often resulting from factors like certain medications, trauma to a tooth, or developmental issues. While option D specifically emphasizes the presence of stains in plaque and calculus as well as on the tooth surface, it also encompasses the understanding that stains can affect the tooth's external surface directly.

Recognizing both locations for stains is crucial for effective dental hygiene and treatment. By identifying the presence of stains both within plaque and calculus and directly on the tooth surface, dental professionals can better formulate cleaning and treatment strategies to improve the patient's oral health.