**Core Concept**
The initial step in root canal therapy involves the insertion of a specific instrument to facilitate access and preparation of the canal for further treatment. This process requires careful consideration of the root canal anatomy and the instruments used to navigate the canal's complex structure.

**Why the Correct Answer is Right**
The first instrument inserted into the root canal is typically a **gutta-percha carrier**, but more specifically in the context of this question, it is the **gates glidden drill**. The gates glidden drill is used to enlarge the root canal and remove the dentin, allowing for the introduction of other instruments. It is designed with a unique cutting edge that helps to remove the dentin and prepare the canal for the next steps in the treatment. The gates glidden drill is inserted through the access cavity to facilitate this initial enlargement.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the **gutta-percha carrier** is typically used to transport and deliver the gutta-percha filling material, not as the initial instrument inserted into the root canal.
* **Option B:** This option is incorrect because the **endodontic file** is used to shape and clean the root canal, but it is not the first instrument inserted into the canal.
* **Option C:** This option is incorrect because the **root canal explorer** is used to assess the canal's patency, but it is not the instrument used to enlarge the canal.

**Clinical Pearl / High-Yield Fact**
When navigating the root canal, it is essential to remember that the gates glidden drill is typically used in a sequence of instruments, including the K-type files and the Hedstrom files, to achieve the desired canal shape and size.

**Correct Answer: C. Gates Glidden Drill**