For a significant external bleeding wound, what is the immediate and most crucial step to control blood loss before professional medical help arrives?

✓ Correct Answer: Option C

Applying direct, firm pressure to the wound with a clean cloth helps to compress the blood vessels and allow clotting to occur, which is the most effective immediate action to control significant external bleeding. Elevation can be helpful but is secondary to direct pressure, and a tourniquet is typically a last resort for severe, uncontrollable bleeding.