**Core Concept**
Cushing's syndrome is a clinical disorder caused by excessive exposure to cortisol, which can be due to various reasons including endogenous production, exogenous administration, or ectopic secretion. In this case, the patient's raised ACTH levels not suppressed by dexamethasone suggest an endogenous cause, specifically a pituitary or ectopic source.

**Why the Correct Answer is Right**
The patient's symptoms of hypertension and obesity are consistent with Cushing's syndrome. The raised ACTH levels not suppressed by dexamethasone indicate an autonomous production of ACTH, which is typically seen in cases of pituitary or ectopic ACTH-producing tumors. The chronic smoking history is a crucial clue, as it increases the risk of lung cancer, which is a common source of ectopic ACTH production.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so no explanation is available.
**Option B:** While adrenal tumors can cause Cushing's syndrome, they are typically characterized by low ACTH levels due to negative feedback from high cortisol levels.
**Option C:** Ectopic ACTH-producing tumors, such as small cell lung cancer, are a more common cause of Cushing's syndrome with autonomous ACTH production than pituitary tumors.
**Option D:** This option is not provided, so no explanation is available.

**Clinical Pearl / High-Yield Fact**
Remember the "triple threat" of Cushing's syndrome: hypertension, obesity, and hyperglycemia. Also, be aware that ectopic ACTH-producing tumors are often associated with smoking and can be a challenge to diagnose.

**Correct Answer:** C.