The volume of water in a cell governed mainly by the amount of ____ in the cytoplasm. is it protein, NA+, or K+ ?

Potassium (K+) and sodium (Na+), both ions of positive charge, or cations, both regulate the amount of water moving in and out of cells.  Water moves across the cell membrane in either direction, entering or leaving the cell; which direction it goes is contingent upon the relative concentrations of these two ions.  Sodium is an extracellular (outside the cell) cation, whereas potassium is an intracellular (inside the cell) cation.