A solution is a homogeneous mixture where one substance (the solute) is completely dissolved in another (the solvent).
Among the options you provided:
a. Salt and water - This is a solution because salt dissolves completely in water, resulting in a homogeneous mixture.
b. Sand and water - This is not a solution because sand does not dissolve in water; it settles at the bottom, creating a heterogeneous mixture.
c. Salad dressing - This can be a mixture, but it is usually not a solution as it typically contains oil and vinegar, which do not completely mix and remain separate (especially if it's an oil-based dressing).
d. Toothpaste - This is generally a paste or gel and contains various ingredients but is not a solution in the traditional sense as it does not consist of one substance dissolved in another.
Thus, the correct answer is a. Salt and water.
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