In the above reaction, the reagent “\( A \)” is:
\( \mathrm{HCl,\ Zn\!-\!Hg} \)
\( \mathrm{NaBH_4,\ H_3O^+} \)
\( \mathrm{Alkaline\ KMnO_4,\ H^+} \)
\( \mathrm{LiAlH_4} \)