The half life periods of four reactions labelled by A, B, C & D are $30\sec ,4.8 \min ,180\sec$ and $16 \min$, respectively. The fastest reaction is

Half life period: The time taken for half of the reaction to be completed i.e, the time in which the concentration of a reactant is reduced to half of its original value. Because the half life period is less for reaction A, so the fastest reaction is A.