**Core Concept**
A well-defined lesion in the lung with popcorn calcification on X-ray is suggestive of a specific type of lung tumor characterized by the presence of calcifications resembling popcorn. This radiographic feature is indicative of a benign tumor type.

**Why the Correct Answer is Right**
The correct answer is a hamartoma, a benign lung tumor composed of a mixture of mesenchymal and epithelial elements. The popcorn calcification is due to the presence of dystrophic calcification within the tumor, which is a characteristic feature of hamartomas. The calcifications are often described as having a "popcorn" or "ground-glass" appearance on X-ray.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not describe a common lung tumor with popcorn calcification. While some other lung tumors may exhibit calcification, it is not typically described as popcorn-like.

**Option B:** This option is incorrect because it is a type of lung cancer that often presents with a different radiographic appearance, such as a mass or nodules.

**Option C:** This option is incorrect because it is a type of lung disease that often presents with different radiographic features, such as fibrosis or nodules.

**Clinical Pearl / High-Yield Fact**
The presence of popcorn calcification in a lung lesion is highly suggestive of a hamartoma, and this feature should be considered when interpreting chest X-rays.

**Correct Answer: D. Hamartoma**