which of the following is principal cation of blood plasma

1. Mg⁺²
2. k⁺
3. Na⁺²
4. Ca⁺²

The principal cation of blood plasma is (3) Na²⁺

Sodium is the most abundant electrolyte, and the most abundant cation, in blood plasma.
Sodium plays a large role in maintaining water balance in the blood and in the tissues. The body monitors sodium and the volume of blood; sensors in certain parts of the body, such as the blood vessels and kidney, tell the kidneys whether to increase or decrease the excretion of sodium and water.

Sodium also plays an important role in the functioning of nerve and muscle cells. Sodium and potassium traveling back and forth across the membranes of cells generate a charge, which can cause a muscle cell to contract or a nerve cell to transmit a signal.