Which of the following is NOT a strong acid?

Water, H₂O, is not classified as a strong acid. Strong acids are substances that completely dissociate in water to yield H⁺ ions, leading to a high concentration of hydrogen ions in solution. In contrast, water is a neutral substance that does not completely ionize to produce H⁺ ions in significant quantities.

When considering the other options, hydrochloric acid (HCl), hydrobromic acid (HBr), and nitric acid (HNO₃) all completely ionize in aqueous solutions, making them strong acids. This complete dissociation is crucial, as it means that the concentration of hydrogen ions in the solution is equal to the concentration of the acid. Hence, water stands out as the only option that does not exhibit this behavior, affirming its classification as a neutral compound rather than a strong acid.