The sodium concentration of the blood is primarily regulated by the

Kidneys

Explanation:

Sodium is an electrolyte which when present in the blood flows in its ionic form. The presence of the sodium ions in blood maintains the osmolarity of the blood by affecting the reabsorption and excretion of the water.

The sodium concentration in blood is maintained by the kidneys, a pair of the bean-shaped organ in the abdomen.

The kidneys when sense the low concentration of the sodium in the blood allows the adrenal glands to secrete aldosterone hormone. The hormone acts on the tubule of the kidney and allows the reabsorption and retention of the sodium in the blood.

Thus, the kidney is the correct answer.