Amount of oxalic acid present in a solution can be determined by its titration with $$KMnO_4$$ solution in the presence of $$H_2SO_4$$. The titration gives unsatisfactory result when carried out in the presence of HCl, because HCl

During the titration of oxalic acid with $$KMnO_4$$, a strongly acidic medium is needed because $$MnO_4^-$$ gets reduced smoothly only in the presence of excess $$H^+$$. Dilute $$H_2SO_4$$ is normally chosen because its anion ($$SO_4^{2-}$$) does not participate in any side redox reaction.

If, instead, $$HCl$$ is used to supply the required $$H^+$$ ions, chloride ions $$\left(Cl^- \right)$$ act as a reducing agent toward the powerful oxidiser $$MnO_4^-$$. The principal side reaction is

$$2 \, MnO_4^- + 10 \, Cl^- + 16 \, H^+ \rightarrow 2 \, Mn^{2+} + 5 \, Cl_2 + 8 \, H_2O$$

Thus, even before oxalic acid can be oxidised, a part of the permanganate is consumed (reduced from $$+7$$ to $$+2$$ oxidation state). Because the titrant is lost in this parasitic reaction, the observed end-point is reached prematurely, giving an erroneously low value for the oxalic-acid concentration.

Hence the titration becomes unsatisfactory in the presence of $$HCl$$ because $$Cl^-$$ ions reduce permanganate to $$Mn^{2+}$$.

Option C which is: reduces permanganate to $$Mn^{2+}$$