**Core Concept**
Urolithiasis, or kidney stones, can be classified based on their composition, with radio-opacity being a key characteristic. Radio-opaque stones are visible on plain radiographs due to their ability to absorb X-rays, whereas radio-lucent stones are not visible.

**Why the Correct Answer is Right**
Approximately 80-90% of renal stones are radio-opaque, primarily composed of calcium oxalate, uric acid, or struvite. These stones contain minerals that absorb X-rays, making them visible on plain radiographs. The high percentage of radio-opaque stones is due to the prevalence of calcium oxalate stones, which are the most common type of kidney stone.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately reflect the percentage of radio-opaque renal stones.
* **Option B:** This option is incorrect because it is too low; the actual percentage of radio-opaque stones is significantly higher.
* **Option C:** This option is incorrect because it is too high; the actual percentage of radio-opaque stones is not as high as 100%.
* **Option D:** This option is incorrect because it is not a specific percentage and does not accurately reflect the percentage of radio-opaque renal stones.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the composition of kidney stones can vary depending on several factors, including diet, medical conditions, and family history. A thorough medical history and laboratory evaluation are crucial in determining the underlying cause of kidney stones.

**Correct Answer: C. Up to 90%**