**Core Concept**
Cholesterol gallstones are the most common type of gallstones, and their formation is influenced by a combination of genetic, hormonal, and environmental factors. The formation of gallstones involves an imbalance between the concentration of bile salts, cholesterol, and phospholipids in the gallbladder.

**Why the Correct Answer is Right**
The formation of cholesterol gallstones is enhanced by factors that increase the concentration of cholesterol in the bile or decrease the concentration of bile salts. For example, a low-fat diet can lead to decreased bile salt secretion, resulting in an increased concentration of cholesterol in the bile. Similarly, conditions that increase the secretion of cholesterol into the bile, such as liver disease, can also contribute to the formation of gallstones.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a high-fat diet can actually decrease the concentration of cholesterol in the bile, making it less likely to form gallstones.
**Option B:** This option is incorrect because a low-fat diet can lead to decreased bile salt secretion, resulting in an increased concentration of cholesterol in the bile.
**Option C:** This option is incorrect because conditions that increase the secretion of cholesterol into the bile, such as liver disease, can contribute to the formation of gallstones.

**Clinical Pearl / High-Yield Fact**
A low-fat diet is often recommended for individuals at risk of developing gallstones, as it can help to decrease the concentration of cholesterol in the bile and reduce the risk of stone formation.

**Correct Answer: A. A high-fat diet.**