those are defined by position of double bonds,

the possible one,s are cis-cis,cis-trans,trans-trans.

The cis-trans isomerism tend to be very stable. Typically, trans isomers are more stable however, an exception lies in cis-trans isomers which makes them more stable than trans isomers.

Yes: cis-3-hexene and trans-3-hexene.

This is a single bonded compound and cis-trans isomerism is only possible when fre rotation about a bond is restricted or a double bond or cycle is essential for existence of cis-trans isomers.

Rotation restricted