**Core Concept**
The question is testing the knowledge of the risk factors associated with thromboembolism. Thromboembolism occurs when a blood clot forms in a blood vessel and breaks loose, traveling to a distant site in the body where it blocks blood flow. This can lead to serious complications, including heart attack, stroke, and pulmonary embolism.

**Why the Correct Answer is Right**
The risk of thromboembolism is highest in conditions where blood flow is stagnant, and the blood is more likely to clot. One such condition is **deep vein thrombosis (DVT)**, which occurs when a blood clot forms in a deep vein, usually in the legs. The risk of thromboembolism is also increased in conditions with hypercoagulability, such as **thrombophilia**, which is a disorder that increases the risk of blood clots.

**Why Each Wrong Option is Incorrect**
**Option A:** **Pulmonary embolism** is a type of thromboembolism, but it is not the condition where the risk of thromboembolism is highest. Pulmonary embolism occurs when a blood clot breaks loose and travels to the lungs, but the risk of this occurring is not as high as in conditions where blood clots form in deep veins.
**Option B:** **Atrial fibrillation** is a cardiac arrhythmia that increases the risk of thromboembolism, but it is not the condition where the risk of thromboembolism is highest. Atrial fibrillation increases the risk of blood clots forming in the heart, but the risk of thromboembolism is not as high as in conditions with deep vein thrombosis.
**Option C:** **Hypertension** is a risk factor for many cardiovascular diseases, including thromboembolism, but it is not the condition where the risk of thromboembolism is highest. Hypertension increases the risk of blood clots forming, but the risk of thromboembolism is not as high as in conditions with deep vein thrombosis.
**Option D:** **Varicose veins** are a common condition where the veins are enlarged and twisted, but they are not typically associated with a high risk of thromboembolism. While varicose veins can increase the risk of blood clots forming, the risk is not as high as in conditions with deep vein thrombosis.

**Clinical Pearl / High-Yield Fact**
The risk of thromboembolism is highest in conditions with **stagnant blood flow**, such as deep vein thrombosis. This is because stagnant blood is more likely to clot, and the risk of blood clots breaking loose and traveling to distant sites in the body is increased.

**Correct Answer:** C. Varicose veins.