If you heat ice, it __________ into water.

The correct answer is A: melts. This sentence exemplifies a zero conditional structure, which is used to describe general truths, scientific facts, or situations where the outcome is always the same. The structure for the zero conditional is 'If + present simple, present simple'. In this case, 'If you heat ice' (present simple) leads directly to 'it melts into water' (present simple). This grammatical rule highlights a cause-and-effect relationship that is universally true. Other options like 'melted' (past tense), 'will melt' (future tense, used for first conditional or predictions), or 'would melt' (conditional, often for hypothetical situations) are incorrect because they do not fit the context of a universal truth. The consistent and predictable nature of ice turning into water when heated makes the simple present tense the only appropriate choice for both clauses in this type of conditional sentence.