Explanation

(d)
Autocatalysis is a process in which one of the product behaves as a catalyst.
$e.g.,2{KMnO}_{4} + 5H_{2}C_{2}O_{4} + 3H_{2}{SO}_{4} \rightarrow K_{2}{SO}_{4} + 2{MnSO}_{4} + 8H_{2}O + 10{CO}_{2}$
(acts as catalyst)
The pink colour of ${KMnO}_{4}$dissappears slowly on reaction with oxalic acid, but the rate of disappearance of colour fastens after sometime due to the formation of ${MnSO}_{4}$ which acts as autocatalyst for the reaction.