Concept:A mixture is a physical combination of two or more substances that retain their own properties and can be separated by physical means.
Explanation:Sodium sulphate,
Na2SO4, is a pure compound formed by chemical bonding.
Hydrogen chloride,
HCl, is a pure compound of hydrogen and chlorine.
Ammonia,
NH3, is a pure compound made of nitrogen and hydrogen.
Salt solution is prepared by dissolving salt,
NaCl, in water,
H2O, without any chemical reaction.
Therefore, salt solution is a mixture because its components can be separated by evaporation.
Answer:D. salt solution