Answer : cis-2-pentene
Explanation : The stereoisomers are the compound which differs only in the spatial arrangement of the atoms across the stereogenic centers.
There are mainly two forms of stereoisomers cis and trans. Cis-isomers are those where the two bulky or functional groups are arranged on the same side across the stereogenic center; whereas, in trans-isomers the two bulky groups or functional groups are arranged in the opposite sides of stereogenic center.