Answer
The first and fourth structures are $cis-but-2-ene$. The second and fifth structures are $but-1-ene$. The third structure is $trans-but-2-ene$.
Work Step by Step
The first and fourth structures are $cis-but-2-ene$. The second and fifth structures are $but-1-ene$. The third structure is $trans-but-2-ene$. The compound is determined by how the atoms are connected to each other.