Which of the following describes a strong acid?

A strong acid is best described as an acid that ionizes completely in solution. This means that when a strong acid is dissolved in water, it dissociates entirely into its constituent ions, leading to a maximum concentration of hydrogen ions (H⁺) in solution. As a result of this complete ionization, strong acids are associated with very low pH values, typically below 3, which is a direct consequence of having a high concentration of free H⁺ ions present.

Because both complete ionization and low pH are characteristic of strong acids, the answer indicating both these elements is accurate. This understanding is crucial in differentiating strong acids from weak acids, which only partially ionize and result in higher pH values due to lower H⁺ concentrations. Understanding these properties is fundamental in studying acid-base chemistry and predicting the behavior of various acids in chemical reactions and solutions.