Answer:
D
Explanation:
Stereoisomers are molecules that have the same molecular formula but different 3-d arrangement of atoms. Three types exist but we are focusing on geometrical cis-trans isomerism here. In a single bond, the molecules can rotate around fixed points and there is no fixed 3-d geometry. In butene and other double-bond-containing molecules (the double bond is signified through the "-ene" suffix and the "-2-" indicates that the double bond begins on the second carbon in the chain), rotational motion is stopped by the double bond. This causes two different types of molecules to occur in nature: cis-2-butene, in which the carbon atom in the chain is on the same side at both ends of the double bond, and trans-2-butene, in which the carbon atom is on opposite sides (like a beach chair if the seat was a double bond). So the stereoisomer of trans-2-butene is cis-2-butene, which is Option D.
