Which one of the following salts when dissolves in water hydrolyse?

Options

A. `NaCI`
B. `NH_(4)CI`
C. `KCI`
D. `Na_(2)SO_(4)`

Explanation

To determine which salt hydrolyzes when dissolved in water, we need to analyze the nature of each salt based on the acids and bases from which they are derived.

### Step-by-Step Solution:

1. **Understanding Hydrolysis**: Hydrolysis occurs when a salt reacts with water to form an acidic or basic solution. Salts derived from strong acids and strong bases do not hydrolyze, while those from weak acids or weak bases do.

2. **Analyzing Each Option**:
- **Option A: NaCl**
- NaCl is formed from NaOH (a strong base) and HCl (a strong acid).
- Since both components are strong, NaCl does not hydrolyze in water.

- **Option B: NH4Cl**
- NH4Cl is formed from NH4OH (ammonium hydroxide, a weak base) and HCl (a strong acid).
- Since one component is a weak base, NH4Cl will hydrolyze in water, producing NH4+ ions and Cl- ions, resulting in an acidic solution.

- **Option C: KCl**
- KCl is formed from KOH (a strong base) and HCl (a strong acid).
- Similar to NaCl, both components are strong, so KCl does not hydrolyze in water.

- **Option D: Na2SO4**
- Na2SO4 is formed from NaOH (a strong base) and H2SO4 (a strong acid).
- Again, both components are strong, so Na2SO4 does not hydrolyze in water.

3. **Conclusion**: Among the given options, only NH4Cl hydrolyzes in water.

### Final Answer:
The salt that hydrolyzes when dissolved in water is **NH4Cl** (Option B).