What ion is at a higher concentration in the extracellular fluid than in the intracellular fluid?

There are two types of fluid in the body extracellular fluid and intracellular fluid (ECF and ICF), together they are account for total body water.
The Sodium (Na+) ion is at higher concentration in the extracellular fluid than in the intracellular fluid. The function of extracellular fluid is that it provide cells to watery environment so that they can easily live and perform their function.

Answer:   d.  carbon dioxide.