Ammonium ions act as a Brønsted-Lowry acid in the reaction.

Steps:

• Identify the reaction: NH₄Cl + Ca(OH)₂ → CaCl₂ + NH₃ + H₂O, where NH₄⁺ reacts with OH⁻.
• Recognize NH₄⁺ donates a proton (H⁺) to OH⁻, forming NH₃ and H₂O.
• Apply Brønsted-Lowry definition: acids donate protons.
• Conclude NH₄⁺ functions as the acid, driving the neutralization.

Why A is correct:

• NH₄⁺ is a Brønsted-Lowry acid because it donates H⁺ to OH⁻ (NH₄⁺ + OH⁻ → NH₃ + H₂O).

Why the others are wrong:

• B: NH₄⁺ accepts a proton in reverse reactions but donates here, not acting as a base.
• C: No electron transfer or oxidation occurs; it's an acid-base reaction.
• D: No reduction or electron donation by NH₄⁺; irrelevant to the process.

Final answer: A