**Core Concept**
The question pertains to the risk of malignancy associated with cold nodules, which are thyroid nodules that do not take up radioactive iodine during a thyroid scan. Cold nodules can be benign or malignant, and evaluating their risk of malignancy is crucial for determining the appropriate management strategy.

**Why the Correct Answer is Right**
Cold nodules have a risk of malignancy that varies depending on their characteristics. Studies have shown that approximately 20% of cold nodules are malignant. The risk of malignancy is higher in nodules with suspicious features such as irregular margins, microcalcifications, and a history of radiation exposure. Fine-needle aspiration biopsy (FNAB) is often used to evaluate the cytological characteristics of cold nodules and guide further management.

**Why Each Wrong Option is Incorrect**
**Option A:** 5% is too low, as most studies have reported a higher risk of malignancy in cold nodules.
**Option B:** 15% is still lower than the actual risk, and may not accurately reflect the proportion of malignant cold nodules.
**Option D:** 40% is excessively high, as it would imply that most cold nodules are malignant, which is not the case.

**Clinical Pearl / High-Yield Fact**
When evaluating cold nodules, it's essential to consider the patient's history, physical examination findings, and imaging characteristics to determine the risk of malignancy. A high index of suspicion is necessary, especially in patients with a history of radiation exposure or those with suspicious clinical features.

**✓ Correct Answer: C. 20%**