To prepare for trimming teeth on a stone cast for an immediate denture, what must you mark?

Marking a line 3mm above the free gingival margin is essential for trimming teeth on a stone cast for an immediate denture. This line serves as a reference point to ensure that the artificial teeth are positioned correctly in relation to the patient's gingival anatomy. By establishing this 3mm margin, the clinician avoids exposing the artificial teeth to the gingiva during initial placement, which could lead to discomfort or improper fit.

In addition to providing a comfortable initial fitting, this marking also plays a crucial role in achieving an aesthetic outcome for the denture. It helps to mimic the natural emergence profiles of teeth while maintaining a proper distance above the gingival margin to prevent irritation or other complications associated with the immediate denture placement.

Other options, while they may seem relevant in a different context, do not provide the necessary guidelines for ensuring that the artificial teeth are appropriately positioned above the existing gingival line, thus highlighting the importance of the 3mm marking in the preparation process.