Concept:Entropy measures the degree of disorder or randomness in a system.Explanation:Gases have high entropy because their particles are widely spread and move randomly.Solids have low entropy because their particles are arranged in an ordered, fixed structure.In the reaction NH3(g)+HCl(s)→NH4Cl(s), a gas is converted into a solid product.The system changes from a highly disordered gaseous state to a more ordered solid state.A decrease in disorder means the entropy change is negative.Answer:C. negative