**Core Concept**
The development of a tooth involves a complex process of tissue formation, starting from the inner layer of the oral ectoderm. The process involves the sequential formation of enamel, dentin, cementum, and pulp tissues. The correct sequence and timing of these tissues are crucial for a healthy tooth.

**Why the Correct Answer is Right**
The first formed hard tissue in the tooth is dentin. Dentin formation begins before the enamel formation, and it is produced by odontoblasts, which are specialized cells that differentiate from the dental papilla. The dentin matrix is secreted by the odontoblasts and mineralized by inorganic ions, resulting in a hard, calcified tissue that provides the framework for the tooth. Enamel formation, on the other hand, is initiated after the dentin formation and is produced by ameloblasts, which are specialized cells that differentiate from the oral ectoderm.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because enamel is not the first formed hard tissue in the tooth. Enamel formation begins after the dentin formation.
**Option B:** This option is incorrect because cementum is a thin layer of bone-like tissue that covers the root of the tooth and is not the first formed hard tissue.
**Option C:** This option is incorrect because pulp is a soft tissue that contains blood vessels, nerves, and connective tissue, and is not a hard tissue.

**Clinical Pearl / High-Yield Fact**
The sequence of hard tissue formation in the tooth is dentin, then enamel, and finally cementum. Remembering this sequence can help in understanding the development and structure of the tooth.

**Correct Answer: D. Dentin**