Which ion serves as counterion to sodium in extracellular fluid?

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The main idea is electroneutrality in extracellular fluid. Sodium is the dominant extracellular cation, and its principal counterion is chloride, the main extracellular anion. Together Na+ and Cl− help balance charges and maintain osmotic stability in plasma and interstitial fluid. While bicarbonate also carries a negative charge and participates in buffering, it is not the primary counterion to sodium. Potassium is mainly intracellular, and magnesium is a separate cation, not the primary balancing anion for sodium. So chloride best fits the role of the counterion to sodium in extracellular fluid.