The correct answer is Zinc oxide.

- Gutta-percha, commonly used in endodontic therapy for root canal fillings, is inherently radiolucent, meaning it does not appear clearly on dental radiographs.
- To enhance its visibility, radiopaque substances are added.
- Among these, zinc oxide is the primary ingredient responsible for increasing the radiopacity of gutta-percha.
- This allows clinicians to effectively visualize the root canal filling during radiographic examination, ensuring proper placement and length of the material.

- While other substances like barium sulfate, calcium carbonate, and titanium dioxide are used in various dental materials for radiopacity or other properties, in gutta-percha formulations, zinc oxide serves as the main radiopacifier.
- Its high atomic number enables it to attenuate X-rays, making the material visible on radiographs.

Reference: Grossman’s Endodontic Practice, 13th Edition, Chapter 9: Materials and Techniques in Root Canal Therapy, Page 248