The primary action of diuretics such as furosemide is to inhibit sodium reabsorption in the thick ascending limb of the loop of Henle. This inhibition decreases sodium and chloride reabsorption, leading to increased excretion of sodium, water, and potassium. Furosemide blocks the Na⁺-K⁺-2Cl⁻ symporter, which results in a potent diuretic effect. It does not stimulate potassium channels; instead, it may cause potassium loss due to increased distal sodium delivery. Therefore, the correct primary mechanism is the inhibition of sodium reabsorption.

Reference: Goodman & Gilman's The Pharmacological Basis of Therapeutics, Brunton et al., 13th Edition.