**Core Concept**
Apexification is a dental procedure used to induce root formation in immature teeth with an open apex, particularly when pulp necrosis occurs due to trauma or caries. It is indicated in permanent teeth with an open apex and a necrotic pulp, where root development is still ongoing.

**Why the Correct Answer is Right**
Apexification uses calcium hydroxide or other agents to stimulate continued root development in immature permanent teeth with an open apex. In this case, the maxillary central incisor has a necrotic pulp and an incompletely formed root, indicating the apex is still open. Calcium hydroxide promotes apical closure and root formation by creating a calcified barrier at the apex, enabling the tooth to develop a mature root structure. This is especially effective in children under 10 years of age when the apex is still open.

**Why Each Wrong Option is Incorrect**
Option A: Formocresol pulpotomy is outdated and carries a high risk of toxicity, including systemic absorption and carcinogenic potential; it is not recommended in modern dentistry and is contraindicated in permanent teeth with open apexes.
Option B: Root canal therapy is not indicated because the apex is still open and root development is ongoing. Performing a root canal in such teeth would prevent natural root formation and is not appropriate.
Option C: Calcium hydroxide pulpotomy is used in primary teeth or for pulp protection, not to induce root formation in immature permanent teeth. It does not promote apical closure or root development effectively.

**Clinical Pearl / High-Yield Fact**
In permanent teeth with an open apex and pulp necrosis, apexification is the treatment of choice to allow natural root development. Always confirm apex status via radiograph before choosing treatment—open apex = apexification, closed apex = root canal therapy.

✓ Correct Answer: D. Apexification