Which of the following compounds will show $cis-trans$ isomerism?
$(i)$ $(CH_3)_2C=CH-C_2H_5$
$(ii)$ $CH_2=CBr_2$
$(iii)$ $C_6H_5CH=CH-CH_3$
$(iv)$ $CH_3CH=CClCH_3$

(C) For a compound to show $cis-trans$ isomerism,each carbon atom of the double bond must be attached to two different groups.
$(i)$ $(CH_3)_2C=CH-C_2H_5$: The first carbon has two identical $CH_3$ groups. No $cis-trans$ isomerism.
$(ii)$ $CH_2=CBr_2$: The first carbon has two identical $H$ atoms. No $cis-trans$ isomerism.
$(iii)$ $C_6H_5CH=CH-CH_3$: The first carbon is attached to $H$ and $C_6H_5$; the second carbon is attached to $H$ and $CH_3$. Both carbons have different groups. Shows $cis-trans$ isomerism.
$(iv)$ $CH_3CH=CClCH_3$: The first carbon is attached to $H$ and $CH_3$; the second carbon is attached to $Cl$ and $CH_3$. Both carbons have different groups. Shows $cis-trans$ isomerism.
Therefore,$(iii)$ and $(iv)$ show $cis-trans$ isomerism.