**Core Concept**
Alpha-fetoprotein (AFP) is a tumor marker that is elevated in certain types of cancers, particularly hepatocellular carcinoma and hepatoblastoma. In children, hepatoblastoma is the most common malignant liver tumor, and AFP is a crucial diagnostic and monitoring tool for this condition.

**Why the Correct Answer is Right**
In hepatoblastoma, AFP is produced by the malignant cells, leading to its elevation in the blood. The sensitivity and specificity of AFP for hepatoblastoma in children are quite high, making it a valuable diagnostic marker. Studies have shown that nearly 90% of children with hepatoblastoma have elevated AFP levels, making this a critical indicator for early detection and treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** 10% is too low, as a significant proportion of children with hepatoblastoma have elevated AFP levels.
**Option B:** 50% is also incorrect, as evidence suggests a higher percentage of children with hepatoblastoma have elevated AFP.
**Option D:** 100% is incorrect, as there are cases of hepatoblastoma where AFP levels are within the normal range.

**Clinical Pearl / High-Yield Fact**
It's essential to note that while AFP is a useful marker for hepatoblastoma, it's not exclusive to this condition. Other conditions, such as germ cell tumors and non-seminomatous tumors, can also elevate AFP levels. Therefore, a diagnosis of hepatoblastoma should be based on a combination of clinical, radiological, and laboratory findings.

✓ Correct Answer: C. 90%