**Core Concept**
Bilirubin is a pigment produced from the breakdown of heme-containing proteins like hemoglobin. To be excreted, bilirubin needs to be converted into a water-soluble form, which is then excreted by the kidneys or liver into the bile. This conversion process involves several enzymes and reactions.

**Why the Correct Answer is Right**
The conversion of bilirubin into its water-soluble form, known as conjugation, occurs primarily in the liver. Here, bilirubin is conjugated with glucuronic acid by the enzyme **UDP-glucuronyltransferase** to form **bilirubin glucuronide**. This conjugation reaction is essential for bilirubin to be excreted into the bile and eventually into the intestine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the enzyme or location responsible for bilirubin conversion.
**Option B:** This option is incorrect because it does not accurately describe the process of bilirubin conversion. While the liver is involved, the specific enzyme and reaction are not mentioned.
**Option C:** This option is incorrect because it does not provide a clear or accurate description of the process of bilirubin conversion.

**Clinical Pearl / High-Yield Fact**
UDP-glucuronyltransferase is an enzyme that plays a crucial role in bilirubin conjugation. Deficiencies in this enzyme can lead to **Gilbert's syndrome**, a condition characterized by elevated bilirubin levels and jaundice.

**Correct Answer: C. Liver**