The most stable and firm support for a finger fulcrum is the:

The most stable and firm support for a finger fulcrum is the tooth surface. Using a tooth surface as a fulcrum is essential in dental procedures because it provides a solid and stable base that minimizes movement during tasks such as hand instrumentation or restorative procedures. The hard structure of the tooth allows for accurate control and precision, which is crucial for maintaining the stability of the hand during treatment.

In contrast, the other options, while they may seem like plausible supports, are not as reliable. The cheeks and buccal vestibule are soft tissues that do not provide the same level of stability as a tooth. They can shift or compress under pressure, making them less effective as fulcrum points. The alveolar mucosa is also soft and less stable, as well as being more sensitive. Therefore, using the tooth surface ensures better control, stability, and safety during dental procedures.