For ${\mathrm{CH}}_4+{\mathrm{Cl}}_2\stackrel{\mathrm{h}v}{⟶}$ product, rate of reaction at $\mathrm{t}=0$ is given by rate $=\frac{-\mathrm{d}}{\mathrm{dt}}\left[{\mathrm{CH}}_4\right]=2\times {10}^{-4}$ $\mathrm{M}-{\min }^{-1}$, then rate of the reaction at $\mathrm{t}=20$ $\min$ is (in $\mathrm{M}-{\min }^{-1}$ )

The given reaction is:

${\mathrm{CH}}_4+{\mathrm{Cl}}_2\stackrel{\mathrm{h}v}{⟶}$ product

It is a photochemical reaction so, it is a zero-order reaction.

The rate of the reaction will be written as:

Rate = $\frac{-\mathrm{d}}{\mathrm{dt}}\left[{\mathrm{Cl}}_2\right]=\frac{-\mathrm{d}}{\mathrm{dt}}\left[{\mathrm{CH}}_4\right]=2\times {10}^{-4}$ $\mathrm{M}-{\min }^{-1}$

Therefore, the correct option is (B).