To determine which substance will form a solution when mixed with liquid benzene (\(C_6H_6\)), we need to understand the principle "like dissolves like." This principle implies that substances with similar polarities tend to dissolve in each other. Benzene is a nonpolar solvent, so it will most readily dissolve nonpolar solutes.
Let's evaluate each given substance:
A. \( \text{MgF}_2 (s) \): Magnesium fluoride is an ionic compound and is polar. It is not likely to dissolve well in nonpolar solvents like benzene.
B. \( \text{I}_2 \): Iodine is a nonpolar molecule. Since benzene is also nonpolar, iodine is likely to dissolve in benzene.
C. \( \text{H}_2O (l) \): Water is a polar solvent. It does not mix well with nonpolar solvents like benzene.
D. \( \text{HCl} (g) \): Hydrogen chloride is a polar molecule when dissolved in water, and it is not expected to dissolve well in nonpolar solvents like benzene.
Therefore, the only substance that will form a solution when mixed with liquid benzene is:
[tex]\[ \boxed{B} \, \text{I}_2 \][/tex]
