**Core Concept**
The management of a carious posterior tooth with an undermined cusp involves preserving the remaining tooth structure while eliminating the carious lesion. This requires a conservative approach that balances the need for tooth preservation with the need for thorough caries removal.

**Why the Correct Answer is Right**
In the case of a carious posterior tooth with an undermined cusp, the best treatment option is a preformed metal crown (PMC). This is because a PMC can provide adequate support and protection to the remaining tooth structure, preventing further decay and fracture. The metal crown can be shaped to fit the tooth, covering the undermined cusp and reinforcing the remaining tooth structure. Additionally, PMCs are less invasive than other restorative options, such as full-coverage crowns, making them an ideal choice for preserving tooth structure.

**Why Each Wrong Option is Incorrect**
**Option A:** A stainless steel crown is typically used for primary teeth, not permanent teeth. While it could be used in this scenario, it is not the best choice due to its more invasive nature and potential to damage the permanent tooth structure.
**Option B:** A full-coverage crown is not necessary for a tooth with an undermined cusp, as it would be more invasive than a PMC and may not be justified by the extent of the caries.
**Option C:** A resin-based composite restoration may not provide adequate support and protection to the undermined cusp, and may not be durable enough to withstand the forces of mastication.

**Clinical Pearl / High-Yield Fact**
When managing a carious posterior tooth with an undermined cusp, it is essential to assess the extent of the caries and the remaining tooth structure before selecting a treatment option. A thorough examination, including radiographic evaluation, can help determine the best course of action and ensure the most conservative and effective treatment.

**Correct Answer: D. Preformed metal crown (PMC)**