**Core Concept**
Adrenal incidentalomas are benign tumors of the adrenal gland that are discovered incidentally during imaging studies for unrelated reasons. The most common type of adrenal incidentaloma is a non-functioning adenoma, which is a benign tumor that does not produce excessive hormones.

**Why the Correct Answer is Right**
The correct answer is a non-functioning adenoma because it is the most common type of adrenal incidentaloma. Non-functioning adenomas are typically small, well-circumscribed tumors that do not produce excessive hormones such as cortisol, aldosterone, or catecholamines. They are often discovered in patients undergoing imaging studies for unrelated reasons, such as chest or abdominal CT scans. The pathophysiology of non-functioning adenomas is not fully understood, but they are thought to arise from the adrenal cortex.

**Why Each Wrong Option is Incorrect**
* **Option A:** Pheochromocytoma is a rare type of adrenal incidentaloma that produces excessive catecholamines. It is not the most common type of adrenal incidentaloma.
* **Option B:** Adrenal cortical carcinoma is a rare and aggressive type of adrenal cancer that is not typically discovered incidentally.
* **Option D:** Adrenal metastasis is a type of adrenal tumor that arises from cancer elsewhere in the body, such as the lung or breast. It is not a primary adrenal tumor and is not the most common type of adrenal incidentaloma.

**Clinical Pearl / High-Yield Fact**
The majority of adrenal incidentalomas are non-functioning adenomas, and the risk of malignancy is extremely low. However, all adrenal incidentalomas should be evaluated with imaging studies and hormonal assays to rule out functional tumors.

**Correct Answer: C. Non-functioning adenoma**