Chemistry - Halogen Derivative Question with Solution | TestHub

In the options, they have acidic $\mathrm{\beta }-\mathrm{H}$ so an elimination reaction takes place and alkene will be formed.

And the formed alkene decolourizes the bromine water {$\stackrel{\oplus }{\mathrm{E}}$ addition reaction of an alkene by $\mathrm{HOBr}$} while in $\left(4\right)$ it does not have $\mathrm{\beta }-\mathrm{H}$ so, there is no formation of alkene, hence does not decolourize ${\mathrm{Br}}_2$ water.