Which statement best describes a synthesis reaction?

• A. Two or more substances combine to form a single product.
• B. A single compound breaks down into simpler substances.
• C. An element replaces another element in a compound.
• D. A substance reacts with oxygen to produce heat and light.

Answer: (A)

Synthesis reactions are when two or more substances combine to form a single product. This idea shows up in the statement: multiple starting materials come together to make one compound. An example is hydrogen and oxygen joining to make water, illustrating how simpler substances build up into a single new substance. The other types describe different processes: a single compound breaking down into simpler substances (decomposition), an element replacing another in a compound (displacement), or a substance reacting with oxygen to release heat and light (combustion). So the description focusing on forming one product from several reactants best captures synthesis.