**Core Concept**
Gallstones are small, hard deposits that form in the gallbladder, and they can be classified into different types based on their composition and radiopacity. Radiopacity refers to the ability of a substance to block or absorb X-rays, making it visible on X-ray imaging.

**Why the Correct Answer is Right**
The majority of gallstones are cholesterol stones, which are radiolucent, meaning they do not block or absorb X-rays. However, a smaller percentage of gallstones are pigment stones, which can be radiopaque due to their composition of bilirubin and calcium salts. The exact percentage of radiopaque gallstones can vary depending on the population being studied, but it is generally estimated to be around 10-20%.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely incorrect because it is too low of a percentage. While some gallstones may not be visible on X-ray, a significant portion of them are indeed radiopaque.
* **Option C:** This option is likely incorrect because it is too high of a percentage. While some studies may report a higher percentage of radiopaque gallstones, this is not the most commonly cited estimate in the medical literature.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the type of gallstone can affect its radiopacity. For example, cholesterol stones are generally radiolucent, while pigment stones are often radiopaque. This is an important consideration when interpreting imaging studies.

**Correct Answer: D. 20%**