Explanation: When a lead-acid battery's electrolyte level drops due to evaporation, it is primarily the water (H₂O) that is lost, leaving the sulfuric acid (H₂SO₄) behind. Therefore, replenishing with pure water, such as distilled or deionized water, is crucial. This restores the correct water-to-acid ratio without altering the acid concentration or introducing contaminants. The recommended level, about 1/4 inch above the plates, ensures proper chemical reaction and prevents plate damage. Adding tap water (B) is incorrect because it contains minerals and impurities that can cause corrosion, self-discharge, and premature battery failure by reacting with the battery plates. Using salt water (C) is severely detrimental, as chloride ions are highly corrosive and would rapidly destroy the battery's internal components, rendering it inoperable. Maintaining electrolyte purity is vital for optimal battery performance and longevity, which is essential for reliable amateur radio equipment operation.