**Core Concept**
The clinical success of a cast post and core in a tooth with a non-vital tooth is largely dependent on its ability to restore the tooth's remaining structure while providing adequate support and retention. The post and core should ideally have a minimal diameter to allow for proper coronal seal and prevent leakage, while also being able to withstand occlusal forces.

**Why the Correct Answer is Right**
The single most important predictor of the clinical success of a cast post and core is **the thickness of the remaining coronal structure**. A thicker remaining coronal structure provides a better seal around the post, reduces the risk of leakage, and distributes the occlusal forces more evenly. This is because a thicker coronal structure allows for a more conservative post design, which in turn reduces the risk of tooth fracture and post failure.

**Why Each Wrong Option is Incorrect**
* **Option A:** **The type of cement used** is not the single most important predictor of clinical success. While the type of cement used can affect the durability of the post and core, it is not as critical as the thickness of the remaining coronal structure.
* **Option B:** **The diameter of the post** is also not the single most important predictor. While a smaller post diameter is generally preferred to minimize the risk of tooth fracture, it is not as critical as the thickness of the remaining coronal structure.
* **Option C:** **The material used for the post and core** is not the single most important predictor. While the material used can affect the strength and durability of the post and core, it is not as critical as the thickness of the remaining coronal structure.

**Clinical Pearl / High-Yield Fact**
When evaluating the clinical success of a cast post and core, remember the **"Three Cs"** of post and core design: **Conservative** (minimal post diameter), **Coronal** (adequate coronal seal), and **Cement** ( durable cement selection).

**Correct Answer: C.