Which solute is least likely to cross a membrane by simple diffusion?

Ions of sodium, potassium, calcium and chloride.

Explanation:

Simple diffusion allows very small, non-polar, fat-soluble molecules to cross the plasma membrane into or out of the cell through a concentration gradient. In addition, it is necessary that these molecules cannot have electrical charges, which prevents sodium, potassium, calcium and chloride ions from crossing the plasma membrane, even if they are very small.

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.