Rain water has a $\mathrm{pH}$ of  $5-6$ due to presence of ${\mathrm{H}}^{+}$ions formed by the reaction of rain water with carbon dioxide present in the atmosphere.

${\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+{\mathrm{CO}}_2( \mathrm{g})\rightleftharpoons {\mathrm{H}}_2{\mathrm{CO}}_3$

${\mathrm{H}}_2{\mathrm{CO}}_3\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{HCO}}_3$

So, the correct option is A.