**Core Concept**
The underlying principle being tested is the distinction between **follicular carcinoma** and **follicular adenoma** of the thyroid, based on histopathological features. This differentiation is crucial for determining the appropriate treatment and prognosis.

**Why the Correct Answer is Right**
Since the correct answer is not provided, let's discuss the general approach to differentiating follicular carcinoma from follicular adenoma. The key feature that distinguishes follicular carcinoma from follicular adenoma is the presence of **capsular invasion** or **vascular invasion** in carcinomas.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific details of option A, we cannot directly address why it is incorrect. However, any option that does not involve capsular or vascular invasion would be incorrect.
**Option B:** Similarly, without specifics, we can say that if option B does not relate to the histological differences between follicular carcinoma and adenoma, it would be incorrect.
**Option C:** If option C suggests a feature that is not unique to follicular carcinoma, such as the presence of follicles which can be seen in both adenomas and carcinomas, it would be incorrect.
**Option D:** Any option that does not accurately reflect the diagnostic criteria for follicular carcinoma would be incorrect.

**Clinical Pearl / High-Yield Fact**
A critical point to remember is that the diagnosis of follicular carcinoma is primarily based on the identification of **capsular or vascular invasion**, which is a key distinguishing feature from follicular adenoma.

**Correct Answer:** Correct Answer: D. Capsular or vascular invasion.