Which of the following drug characteristics influences its ability to cross a cell membrane?

The ability of a drug to cross a cell membrane is significantly influenced by its molecular size. Smaller molecules typically have an easier time passing through cell membranes compared to larger ones. This is due to the structure of the lipid bilayer that constitutes the cell membrane, which acts as a selective barrier. Small, non-polar molecules can diffuse more readily across the membrane, while larger or polar molecules may require specific transport mechanisms, such as carrier proteins or channels, to facilitate their passage.

The other options do not have the same level of impact on permeability. The color of the drug is irrelevant to its ability to cross membranes, as color is not related to molecular interactions with lipid layers. The temperature of the environment can affect molecular kinetics and solubility but does not directly determine whether a drug can cross a membrane. Chemical stability may influence a drug's effectiveness or lifespan in the body, but it does not play a direct role in the ability to cross membranes. Thus, molecular size remains the most critical factor in this context.