Step 1: Understand the function of a pitot tube. A pitot tube is an instrument used to measure the velocity of fluid flow. It works based on Bernoulli's principle, which states that the total energy at a point in a moving fluid remains constant. The pitot tube measures stagnation pressure and uses it to calculate the velocity of the fluid. Step 2: Evaluate each option. Pressure: While pressure is involved, it's not the primary measurement. Velocity of flow: Correct; this is the main purpose of a pitot tube. Difference in pressure: This is an intermediate step in measuring velocity. Density: Not measured by a pitot tube. Step 3: Select the correct option. Based on its design and working principle: $$ \boxed{\text{Velocity of flow}} $$