**Core Concept**
The IUB (International Union of Biochemistry) system is a classification of enzymes based on their catalytic reactions. Hydrolases are a group of enzymes that catalyze the hydrolysis of various chemical bonds.

**Why the Correct Answer is Right**
Hydrolases belong to Class 3 in the IUB system, which includes enzymes that catalyze the cleavage of a chemical bond using water. This class includes enzymes such as esterases, lipases, and peptidases that break down ester, lipid, and peptide bonds, respectively. The IUB system categorizes enzymes based on their catalytic reactions, and Class 3 specifically includes enzymes that use hydrolysis as a mechanism to break chemical bonds.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because Class 1 in the IUB system includes oxidoreductases, which catalyze oxidation and reduction reactions, not hydrolysis.
**Option B:** Incorrect because Class 2 in the IUB system includes transferases, which catalyze the transfer of a group from one molecule to another, not hydrolysis.
**Option C:** Incorrect because Class 6 in the IUB system includes ligases, which catalyze the formation of a chemical bond using energy from ATP, not hydrolysis.

**Clinical Pearl / High-Yield Fact**
Remember that the IUB system is an older classification system, and the more commonly used Enzyme Commission (EC) number system has largely replaced it. However, understanding the IUB system can still be helpful in understanding the broader categories of enzyme reactions.

**Correct Answer:** C. Class 3.