**Core Concept**
The question involves the management of pulp extirpation in narrow canals, which requires a thorough understanding of endodontic procedures and materials. Mummifying agents are used to dry out the pulp tissue, making it easier to remove.

**Why the Correct Answer is Right**
A mummifying agent is used to dry out the pulp tissue in narrow canals, allowing for easier removal of the pulp. This is essential in endodontic procedures, especially when the canal is narrow and difficult to navigate. The mummifying agent helps to reduce the moisture content of the pulp, making it less likely to cause bleeding or other complications during the procedure. This allows the dentist to more easily remove the pulp and prepare the canal for further treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** Obtundant is not a term used in endodontics to describe a material used for pulp extirpation. Obtundants are actually used to reduce the sensitivity of a tooth.
**Option B:** Astringent is not used to dry out pulp tissue; instead, it is used to reduce bleeding and inflammation in the oral cavity.
**Option C:** Haemostatic agents are used to control bleeding, but they are not used to dry out pulp tissue or facilitate its removal.

**Clinical Pearl / High-Yield Fact**
When using a mummifying agent, it is essential to follow the manufacturer's instructions and to use the correct amount to avoid overdrying the pulp tissue. This can lead to complications, such as tooth fracture or perforation.

✓ Correct Answer: D. Mummifying agent