which compound will have cis-trans isomers?

Question

CH3CH2CH3
CH2CCH2
CH3CH2CHCHCH3
C6H6

Answer 1

First of all you need a double bond. So the first compound is rejected.
(Alkynes DON'T have cis-trans isomers).
the second molecule has a linear geometry since the middle C atom forms 2 double bonds CH2=C=CH2. So we would be looking at the position of the substituents on C-1 and C-3. However all substtuents are identical thus there are no cis-trans isomers.

The third molecule hasci-trans isomers, since it has a double bond and each C atom carries two groups that are not identical.
CH3CH2 .. .. CH3 .. .. .. CH3CH2 .. .. H
.. .. .. .. .. \ .. ../ .. .. .. .. .. .. . .. .. .. .. \ .. ../
.. .. .. .. .. C=C .. . .. .. .. .. .. .. .. .. .. C=C
.. .. .. .. .. / .. .. \. .. .. .. .. .. .. .. .. .. .. / .. .. \
.. .. .. .. H .. . .. H .. .. .. .. .. .. .. .. .. H .. .. . CH3
.. .. .. .. . . cis .. .. . .. .. .. .. .. .. .. .. .. trans

C6H6 corresponds to benzene. There all 3 bonds have to be cis. Trans isomers cannot exist for benzene due to the torsion of the ring that would be required.
However C6H6 could correspond in theory to other molecules as well, that have degree of unsaturation =4.
So it could be
CH-=C-CH=CH-CH=CH2
where -= is triple bond. This would have cis-trans only for the double bond in the middle.
or CH2=CH-C-=C-CH=CH2. which has no cis-trans isomers for either double bond (each double bond has a C atom with 2 H)

Answer 2

Alkynes and Alkenes may have cis-trans isomers if there is a double or triple bond in the middle of it and also provided that it is not linear as in CH2CCH2. CH3CH2CH3 is an alkane. C6H6 is benzene and can only have its ring structure.

CH3CH2CHCHCH3 is the only one that works.