The correct answer is D) decreased pressure on the hull resulting from increased speed. Squat is a natural phenomenon that occurs when a ship moves through a restricted waterway, such as a channel or shallow water. As the ship's speed increases, the water flow around the hull is disrupted, resulting in a decrease in pressure on the hull. This decrease in pressure causes the ship to "squat," or sink deeper into the water, effectively reducing the under-keel clearance. This principle is based on Bernoulli's principle, which states that as the speed of a fluid increases, the pressure within the fluid decreases. The other options are incorrect because they do not accurately describe the underlying principle behind squat. Increased speed leads to decreased pressure, not increased pressure as suggested in options B and C. Additionally, a decrease in speed would not result in a decrease in pressure on the hull, as stated in option A.