**Core Concept**
Dentinogenesis is the process of dentin formation, and in direct pulp capping cases, the formation of dentinal bridges is crucial for the success of the procedure. This process involves the proliferation and differentiation of odontoblasts, which are specialized cells responsible for dentin production.

**Why the Correct Answer is Right**
The correct answer is related to the role of **calcium hydroxide** in promoting dentinogenesis. Calcium hydroxide is a well-documented material that stimulates the formation of dentinal bridges by inducing the proliferation and differentiation of odontoblasts. It acts by releasing calcium ions, which stimulate the cells and promote the deposition of minerals necessary for dentin formation. This process is also influenced by the presence of growth factors, such as **transforming growth factor-beta (TGF-β)**, which are released by calcium hydroxide and play a crucial role in regulating odontoblast differentiation.

**Why Each Wrong Option is Incorrect**
* **Option A:** While **mineral trioxide aggregate (MTA)** is a biocompatible material used in direct pulp capping, it is not primarily responsible for the formation of dentinal bridges through dentinogenesis.
* **Option B:** **Formocresol** is a material used in vital pulp therapy, but it is not associated with the promotion of dentinogenesis or the formation of dentinal bridges.
* **Option D:** **Thermoplasticized gutta-percha** is a material used in endodontic procedures, but it is not relevant to the process of dentinogenesis or the formation of dentinal bridges.

**Clinical Pearl / High-Yield Fact**
When using calcium hydroxide for direct pulp capping, it is essential to ensure that the material is in contact with the pulp tissue for an adequate period, as this will promote the release of growth factors and stimulate dentinogenesis.

**Correct Answer:** C.