Eliminate incorrect options: Calcium (Ca extsuperscript{2+}) and sodium (Na extsuperscript{+}) are primarily extracellular ions, meaning they are found outside the cell. Similarly, sodium (Na extsuperscript{+}) and chloride (Cl extsuperscript{-}) are also extracellular ions. Potassium (K extsuperscript{+}) and magnesium (Mg extsuperscript{2+}) are not paired with phosphate, making this option incorrect.