**Core Concept**
A solitary pulmonary nodule (SPN) is a common finding on chest CT scans, and its evaluation is crucial to distinguish between benign and malignant lesions. The evaluation of an SPN involves assessing various radiological features that may suggest a benign or malignant nature.

**Why the Correct Answer is Right**
The correct answer suggests that the presence of a well-defined border, a smooth contour, and a diameter of less than 4 cm are features that are more commonly associated with benign lesions. These features are thought to be due to the presence of a fibrotic or calcified capsule around the nodule, which is more characteristic of benign lesions. Additionally, the diameter of less than 4 cm is a risk factor for malignancy, with larger nodules being more likely to be malignant.

**Why Each Wrong Option is Incorrect**
**Option A:** A spiculated or irregular border is often associated with malignant lesions due to the presence of tumor invasion or infiltration into surrounding tissues.
**Option B:** A nodule with an air bronchogram sign (the presence of air-filled bronchi within the nodule) is more commonly seen in malignant lesions, as it suggests the presence of tumor necrosis or cavitation.
**Option C:** A nodule with a diameter of more than 4 cm is more likely to be malignant, as larger nodules are associated with a higher risk of cancer.

**Clinical Pearl / High-Yield Fact**
When evaluating a solitary pulmonary nodule, it's essential to remember the "HALT" criteria, which stands for "Heterogeneity," "Air bronchogram," "Lobulation," and "Tuberculosis" (or "Tram-track" calcification). These features are more commonly associated with malignant lesions, and their presence should prompt further evaluation or biopsy.

**Correct Answer:** C. A nodule with a smooth contour, well-defined border, and diameter of less than 4 cm is more likely to be benign.