Final answer:
Sodium ions (Na+) have the highest extracellular to intracellular concentration ratio in most mammalian cells due to active transport mechanisms like the sodium-potassium pump. Therefore correct option is A
Step-by-step explanation:
The substance with the highest extracellular fluid to intracellular concentration ratio for most mammalian cells is sodium ions (Na+). Sodium is the major cation of the extracellular fluid and plays a significant role in maintaining the osmotic pressure gradient between cells and their environment. While substances such as potassium ions (K+), carbon dioxide (CO2), glucose, and proteins are present within cells, the sodium-potassium pump actively transports sodium out of cells and potassium into the cells, resulting in a higher concentration of sodium in the extracellular fluid compared to the intracellular fluid.