The reaction 2A + B _ 2 → 2AB is an elementary reaction. For a...

Reaction : $2A + {B}_{2} ⟶ 2AB$

As the reaction is elementary, the rate of reaction is

$r = K . [ A{]}^{2}\left[{B}_{2}\right]$

on reducing the volume by a factor of $3,$ the concentrations of $A$ and ${B}_{2}$ will become 3 times and hence, the rate becomes ${3}^{2} \times 3 = 27$ times of initial rate.