[Moneyking]

Cis-trans isomers are possible because of:

a) free rotation about a single bond
b) carbon bonding orbitals are tetrahedral
c) there is no rotation about single bonded carbons
d) there is no rotation about double bonded carbons
e) organic molecules can resonate

Ok, I think it can`t be a,e,or b, i think its either c or d, and final answer is d, because cis-trans are only in alkenes which have 1 double bond

[discodermolide]

d is correct. There is no rotation about C=C.
Note you also have cis/trans isomers in compounds with more than one C=C.

[Sophia7X]

c) there is no rotation about single bonded carbons
d) there is no rotation about double bonded carbons

Not c, there is free rotation around singly bonded carbons.
But double bonded carbons can't freely rotate because double bonds involve pi bonds (as well as sigma bonds), and pi bonds are above and below the internuclear (bond) axis. While single bonded carbons just have a sigma bonds, formed on the internuclear axis.