**Core Concept**
The management of a solitary pulmonary nodule (SPN) involves determining its benign or malignant nature. Radiological features play a crucial role in this differentiation. The benign nature of an SPN can be inferred by specific characteristics on imaging.

**Why the Correct Answer is Right**
The key radiological feature to suggest the benign nature of a solitary pulmonary nodule is calcification. Benign SPNs, such as hamartomas, often exhibit calcification patterns, including fine, coarse, or diffuse calcification. These calcifications are typically punctate or central, and they may be surrounded by a thin rim of soft tissue. This characteristic helps distinguish benign from malignant SPNs.

**Why Each Wrong Option is Incorrect**
**Option A:** Smooth margins are more commonly associated with malignant SPNs, as they tend to have a smooth, well-defined border. This feature is less indicative of a benign SPN.
**Option B:** The presence of a fat density within the nodule is a classic feature of a pulmonary sequestration, which is a congenital anomaly, but not a typical benign SPN.
**Option C:** A spiculated margin is a characteristic of malignant SPNs, indicating invasion into surrounding lung tissue. This feature is less likely to be associated with a benign SPN.

**Clinical Pearl / High-Yield Fact**
When evaluating a solitary pulmonary nodule, remember the "3 Ps": "Partially solid," "Pleural tag," and "Pulmonary artery sign." These features are more commonly associated with malignant SPNs.

**Correct Answer:** D. Calcification.