Explanation

(d)
When Alead storage battery is discharged, the following cell reactions take place.
At anode
Pb + $H_{2}{SO}_{4}\ \longrightarrow \ {PbSO}_{4} + \ {2H}^{+} + \ {2e}^{-}$
At cathode
${\ PbO}_{2} + \ {4H}^{+} + \ {SO}_{4}^{2 -} + \ {2e}^{-}\ \longrightarrow \ {PbSO}_{4} + \ {2H}_{2}O$