**Core Concept**
Gallstones are solid particles that form in the gallbladder, composed of various substances such as cholesterol, bilirubin, and calcium salts. The pathogenesis of gallstones involves an imbalance between the secretion of bile salts and the concentration of bile, leading to the formation of stones.

**Why the Correct Answer is Right**
Cholesterol stones are the most common type of gallstones, accounting for approximately 70-80% of cases. They form when there is an excess of cholesterol in the bile, which is then crystallized out of solution. This occurs when the bile is supersaturated with cholesterol, often due to a decrease in bile salts or an increase in cholesterol production.

**Why Each Wrong Option is Incorrect**
**Option A:** **Pigment stones** are another type of gallstone, but they are primarily composed of bilirubin and are more common in patients with hemolytic disorders or liver disease. Cholesterol stones are distinct from pigment stones in terms of their composition and pathogenesis.
**Option B:** **Calcium stones** are not a specific type of gallstone, but rather a characteristic of certain stones that contain calcium salts. While calcium can be present in some gallstones, this option is too vague and does not specifically refer to a known type of gallstone.
**Option C:** **Mixed stones** are a type of gallstone that contains a combination of cholesterol, bilirubin, and calcium salts. While mixed stones are common, this option is not specific to cholesterol stones, which are the focus of the question.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the formation of cholesterol gallstones is often associated with obesity, diabetes, and other metabolic disorders. These conditions can lead to an increase in cholesterol production and a decrease in bile salts, increasing the risk of gallstone formation.

**Correct Answer: C. Cholesterol stones.**