Correct Answer: 21%

Solution : The correct option is 21%.

According to the chemical formula of ammonium sulphate (NH ₄ ) ₂ SO ₄ ), each molecule of ammonium sulphate includes two ammonium ions (NH ⁴⁺ ). One nitrogen atom (N) is present in each ammonium ion (NH ⁴⁺ ). The molar mass of nitrogen and the molar mass of ammonium sulphate may be used to calculate the proportion of nitrogen in ammonium sulphate: Nitrogen (N) molar mass is about 14 grams/mol. Ammonium sulphate [NH ₄ ) ₂ SO ₄ ] molar mass: (2 NH ₄ ⁺ ions) + (1 SO ₄ - ² ion); (2 ×18 g/mol for NH ₄ ⁺ ) + (1 × 96 g/mol for SO ₄ - ² ); 36 g/mol plus 96 g/mol = 132 g/mol;  the percentage of nitrogen in ammonium sulphate: (2 × Molar mass of nitrogen) / Molar mass of ammonium sulfate; (2 × 14 g/mol) / 132 g/mol; (28 g/mol) / (132 g/mol) ≈ 0.2121
To express this as a percentage, multiply by 100:
0.2121 × 100 ≈ 21.21%