Explanation

(c)
When one of the products of a reaction acts as a catalyst for the reaction, the phenomenon is known as auto catalysis. When ${KMnO}_{4}$solution is added to oxalic acid, the disappearance of pink colour is slow at start but as soon as some ${Mn}^{2 +}$ion are formed the disappearance of colour becomes fast.
$2{MnO}_{4}^{-} + 5C_{2}O_{4}^{2 -} + 16H^{+} \longrightarrow 2{Mn}^{2 +}10{CO}_{2} + 8H_{2}O$
(catalyst)
or $2{KMnO}_{4} + 5H_{2}C_{2}O_{4} + 3H_{2}{SO}_{4} \longrightarrow 2{MnSO}_{4} + K_{2}{SO}_{4} + 8H_{2}O + 10{CO}_{2}$
(catalyst)
${Mn}^{2 +}$ion (or${MnSO}_{4}$)acts as catalyst in this reaction.