**Core Concept**
The underlying principle being tested is the concept of apical seal in root canal treatment, specifically the length of gutta-percha (GP) seal remaining after post space preparation. This is crucial for preventing bacterial leakage and ensuring the long-term success of the endodontic treatment.

**Why the Correct Answer is Right**
The correct answer is based on the recommendation that at least 4-5 mm of apical GP seal should remain to provide an adequate seal against bacterial penetration. This length is considered sufficient to prevent recontamination of the canal and allow for the placement of a post while maintaining the integrity of the apical seal.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not provide sufficient length for an adequate apical seal.
**Option B:** Similarly, this option is too short and would compromise the seal.
**Option D:** This option exceeds the necessary length, potentially leaving insufficient space for post placement.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the apical seal is critical for the success of root canal treatment, and compromising this seal can lead to treatment failure. Ensuring an adequate length of GP seal is vital.

**Correct Answer:** Correct Answer: C. 5 mm.