The correct answer is A. Head of water (h).
The velocity of a jet of water traveling out of an opening in a tank filled with water is proportional to the head of water, which is the vertical distance between the water surface and the opening. This is because the higher the head of water, the greater the pressure difference between the water at the surface and the water at the opening, and the greater the force that will be exerted on the water at the opening, causing it to accelerate.
Option B is incorrect because the square of the head of water is not proportional to the velocity of the jet. Option C is incorrect because the velocity of the jet is not proportional to the volume of water flowing out of the opening per unit time. Option D is incorrect because the velocity of the jet is not proportional to the head of water divided by 2.