The electrolyte which is completely ionised in its aqueous solution is
(a) ethanoic acid
(b) hydrochloric acid
(c) ammonium hydroxide
(d) hydrogen sulphide

Options

A. ethanoic acid
B. hydrochloric acid
C. ammonium hydroxide
D. hydrogen sulphide

Answer

To determine which electrolyte is completely ionized in its aqueous solution, we need to analyze the given options one by one.

### Step-by-Step Solution:

1. **Identify the Electrolytes:**
- (a) Ethanoic acid (CH₃COOH)
- (b) Hydrochloric acid (HCl)
- (c) Ammonium hydroxide (NH₄OH)
- (d) Hydrogen sulfide (H₂S)

2. **Analyze Ethanoic Acid:**
- Ethanoic acid is a weak acid.
- In aqueous solution, it partially ionizes to form CH₃COO⁻ (acetate ion) and H⁺ (hydrogen ion).
- Therefore, it does not completely ionize.

3. **Analyze Hydrochloric Acid:**
- Hydrochloric acid is a strong acid.
- In aqueous solution, it completely dissociates into H⁺ and Cl⁻ ions.
- Therefore, it is a strong electrolyte and completely ionized.

4. **Analyze Ammonium Hydroxide:**
- Ammonium hydroxide is a weak base.
- In aqueous solution, it partially ionizes to form NH₄⁺ (ammonium ion) and OH⁻ (hydroxide ion).
- Therefore, it does not completely ionize.

5. **Analyze Hydrogen Sulfide:**
- Hydrogen sulfide is a weak acid.
- In aqueous solution, it partially ionizes to form H⁺ ions and S²⁻ ions.
- Therefore, it does not completely ionize.

6. **Conclusion:**
- Among the four options, only hydrochloric acid (HCl) completely ionizes in its aqueous solution.
- Thus, the correct answer is **(b) Hydrochloric acid**.

### Final Answer:
(b) Hydrochloric acid