**Core Concept**
Dentinogenesis is the process of dentin formation, a key component of tooth structure. It involves the coordinated action of odontoblasts, which are specialized cells responsible for producing dentin matrix. The process is crucial for tooth development and repair.

**Why the Correct Answer is Right**
Dentinogenesis begins at the cusp tip of the developing tooth, where odontoblasts first differentiate and start producing dentin matrix. This process is mediated by the expression of specific genes and the activation of various signaling pathways. The odontoblasts then continue to produce dentin throughout the tooth's development, gradually moving towards the root.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the process of dentinogenesis does not begin at the root of the tooth.
**Option B:** This option is incorrect as the cells responsible for dentinogenesis are odontoblasts, not ameloblasts, which are involved in enamel formation.
**Option C:** This option is incorrect as dentinogenesis begins before the tooth erupts into the oral cavity.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the process of dentinogenesis is a continuous process that occurs throughout tooth development and repair, and is crucial for maintaining the tooth's structure and function.

**Correct Answer: C. The cusp tip of the developing tooth.**