In the given chemical reaction, H2SO4 is reacting with NaCl to form HCl and NaHSO4. H2SO4 is behaving as a strong acid because it donates a proton (H+) to NaCl to form HCl. This is evident from the fact that H2SO4 dissociates completely in water to form H+ and SO4^2-. Additionally, H2SO4 is not involved in any redox reaction and does not act as a solvent in the given reaction. Therefore, the correct option is "a strong acid."