Which of the following will produce a solution with pH less than 7 at equivalent point?

The substance that will produce a solution with pH less than 7 at the equivalent point is an acid. At the equivalent point, the moles of acid will equal the moles of base added, which means that the pH will be determined by the dissociation of the conjugate acid or base that remains in solution. Out of the given options, HNO3 is a strong acid and NaOH is a strong base, so their reaction will produce a salt (NaNO3) and water, and the solution will have a pH of 7 at the equivalent point. Similarly, H2SO4 is a strong acid and KOH is a strong base, so their reaction will produce a salt (K2SO4) and water, and the solution will have a pH of 7 at the equivalent point. HC is a weak acid, and Mg(OH)2 is a weak base, so their reaction will produce a salt (MgC2O4) and water. However, since HC is a weak acid, some of it will remain undissociated at the equivalent point, resulting in an acidic solution with a pH less than 7. HNO3 is a strong acid and KOH is a strong base, so their reaction will produce a salt (KNO3) and water, and the solution will have a pH of 7 at the equivalent point. Therefore, the substance that will produce a solution with pH less than 7 at equivalent point is HC + Mg(OH)2.