Which of the following is classified as a strong acid?

The classification of an acid as "strong" is based largely on its ability to dissociate completely in water, leading to a high concentration of hydrogen ions (H⁺). The pH scale, which ranges from 0 to 14, measures the acidity of a solution, with lower values indicating higher acidity.

A strong acid will have a very low pH, typically below 3, because it ionizes completely in aqueous solution. In this context, a solution with a pH of 1 represents a very high concentration of hydrogen ions, indicating that it is indeed a strong acid. This is because at pH 1, the concentration of hydrogen ions is approximately 0.1 moles per liter, aligning with the characteristics of strong acids.

In contrast, a pH of 7 is neutral, pH 5 is mildly acidic, and pH 3 is still on the acidic side but not as strong as pH 1. Therefore, among the given choices, the correct answer is the one that indicates the highest acidity, making it the strong acid.