Step-by-step explanation:
Geometric isomerism occurs especially in open-chain compounds that have a double bond between at least two carbon atoms, with each carbon atom of the double having its bonded groups different from each other.
We have:
Cis: Equal linking groups are on the same side of the spatial plane;
Trans: Equal ligand groups are on opposite sides of the spatial plane.
The molecule of question 20 has equal ligand groups on opposite sides of the spatial plane.
It means that is it a trans isomer of 2-butene.
Answer: c. trans-2-butene