**Core Concept**
Solitary pulmonary nodules (SPNs) are a common radiological finding that can be benign or malignant. The features that suggest a benign nature of an SPN are crucial in differentiating it from a malignant one.

**Why the Correct Answer is Right**
Popcorn calcification is a characteristic feature of benign pulmonary hamartomas, which are common benign lung tumors. This type of calcification appears as small, stippled or "popcorn-like" calcifications within the nodule, making it highly suggestive of a benign lesion. The presence of popcorn calcification can significantly reduce the likelihood of a malignant diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** A size of 6cm is not a feature of benign nature of solitary pulmonary nodule; rather, larger size is often associated with malignant lesions.
**Option B:** Acute onset is not a characteristic feature of benign solitary pulmonary nodules; malignant lesions can also present with acute symptoms.
**Option C:** Small specks of calcification can be seen in both benign and malignant lesions, making it a non-specific feature.

**Clinical Pearl / High-Yield Fact**
When evaluating a solitary pulmonary nodule, remember that popcorn calcification is highly suggestive of a benign pulmonary hamartoma. However, always consider the patient's clinical history, imaging characteristics, and any associated symptoms to make an accurate diagnosis.

**Correct Answer: D. Popcorn calcification.**