In the above reaction, the reagent "A" is:

$\mathrm{NaBH}_{4}, \mathrm{H}_{3} \mathrm{O}^{+}$

$\mathrm{LiAlH}_{4}$

Alkaline $\mathrm{KMnO}_{4}, \mathrm{H}^{+}$

$\mathrm{HCl}, \mathrm{Zn}-\mathrm{Hg}$