Structure 1 is a cis-isomer

Structures 1, 2 and 3 are the same compound

5 and 7 are the pair of structures which are enantiomers

The stereochemical relationship between structures 6 and 7 is that they are diastereomers of each other

Structure 4 is a trans-alkene

The meso-structure is 6

6 and 7 are the pair of structures which are enantiomers