**Core Concept**
Calcium hydroxide apexification is a procedure used to induce the formation of a calcific barrier within a tooth with an open apex, allowing for further root development and ultimately enabling obturation. This process relies on the activation of certain cellular pathways and the deposition of calcium salts.

**Why the Correct Answer is Right**
The most reliable sign before obturation is the presence of a calcified barrier, which can be confirmed through radiographic evaluation. This calcified barrier serves as a natural barrier to prevent bacterial invasion and provides a foundation for the deposition of secondary dentin, allowing for the formation of a functional root. The calcification process is often monitored through regular radiographic assessments, which help determine the optimal time for obturation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the presence of a calcified barrier is not the only factor that determines the reliability of the apexification process. While the formation of a calcified barrier is essential, other factors such as the tooth's overall health and the presence of any residual infection also play crucial roles.

**Option B:** This option is incorrect as the presence of a calcified barrier does not necessarily indicate the completion of the apexification process. The process may take several months to a few years to complete, and radiographic evaluation is necessary to determine the optimal time for obturation.

**Option C:** This option is incorrect as the presence of a calcified barrier is not directly related to the tooth's sensitivity. While the formation of a calcified barrier may reduce tooth sensitivity, this is not the most reliable sign before obturation.

**Clinical Pearl / High-Yield Fact**
A key aspect of calcium hydroxide apexification is the formation of a calcific barrier, which serves as a natural barrier to prevent bacterial invasion and provides a foundation for the deposition of secondary dentin.

**Correct Answer:** C. Presence of a calcified barrier