**Core Concept**
Cholesterol stones are the most common type of gallstones, accounting for more than 80% of cases. These stones are primarily composed of cholesterol and other substances, such as bile pigments and calcium salts. The formation of cholesterol stones is influenced by factors like bile composition, gallbladder motility, and lipid metabolism.

**Why the Correct Answer is Right**
The correct answer is **B. Cholesterol**. Cholesterol stones form when there is an imbalance in the bile composition, leading to an increase in cholesterol saturation. This occurs when the bile contains more cholesterol than it can dissolve, causing the excess cholesterol to precipitate out of the solution and form stones. The formation of cholesterol stones is also influenced by genetic factors, such as mutations in the ABCG5 and ABCG8 genes, which regulate cholesterol transport in the liver.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because cholesterol stones are not primarily composed of bile pigments. While bile pigments, such as bilirubin, can be present in gallstones, they are not the main component of cholesterol stones.
**Option C:** This option is incorrect because calcium stones are a different type of gallstone, accounting for around 10% of cases. Calcium stones are primarily composed of calcium salts and are often associated with conditions like primary biliary cirrhosis.

**Clinical Pearl / High-Yield Fact**
A key factor in the formation of cholesterol stones is the presence of supersaturated bile, which occurs when the bile contains more cholesterol than it can dissolve. This can be influenced by factors like a high-fat diet, obesity, and certain medications, such as estrogen replacement therapy.

**Correct Answer: B. Cholesterol**