Which of the following examples illustrates an extrinsic stain?

An extrinsic stain refers to discoloration that occurs on the outer surface of the teeth due to external factors. The correct choice emphasizes common sources of these stains, which are typically found in substances that can come into contact with the teeth. Tobacco, food, and drinks are well-known contributors to extrinsic stains, as they can leave deposits on the enamel and lead to visible discoloration over time.

In contrast, cavity-causing bacteria and tooth decay pertain to the health and integrity of the tooth structure rather than staining. Intrinsic discoloration resulting from genetics involves changes to the internal structure of the tooth, such as variations in the dentin or enamel that are present from the time the teeth develop. Hence, these options do not exemplify extrinsic stains as they focus more on health issues or internal changes rather than surface stains caused by external substances.