Extracellular Fluid (ECF) as it name shows means outside the cell, so it is the fluid which is outside the cell and the fluid which is inside the cells is known as intracellular fluid (ICF).
The predominant extracellular cation is Sodium and Anion is chloride. The intracellular cation is Potassium.
so, the answer to this question is Sodium cation.
