Final answer:
Carbon dioxide is the substance most likely to cross a cell membrane by simple diffusion because it is non-polar and can easily pass through the lipid bilayer without requiring energy or assistance from transport proteins.
Step-by-step explanation:
The movement of substances across cell membranes can occur via various mechanisms, depending on the nature of the substance. Some substances can cross the membrane by simple diffusion, a process that does not require any energy and relies on the molecule's ability to move along a concentration gradient from an area of higher concentration to an area of lower concentration. Substances that are non-polar and without a charge, such as carbon dioxide, can diffuse through the lipid bilayer of the cell membrane easily.
Proteins are generally too large and hydrophilic to diffuse through the membrane without assistance, and thus would not move by simple diffusion. Potassium ions and other charged particles cannot pass through the lipid core of the membrane due to their charge and require specific channels or transporters. Glucose is a polar molecule and also requires a transporter to move across the plasma membrane, through a process known as facilitated diffusion. Therefore, the answer to the question is B. Carbon dioxide, which crosses the cell membrane by simple diffusion.