Geometrical isomerism (cis/trans) requires two different groups on each of the carbons involved in the restricted-rotation feature (double bond or ring).

Pent-1-ene: terminal alkene, one end has two H's — no cis/trans.

2-Methylhex-2-ene: one of the double-bond carbons carries two identical methyl groups — no cis/trans.

1,1-Dimethylcyclopropane: both methyl groups are on the same carbon — no cis/trans.

1,2-Dimethylcyclohexane: the two methyls are on adjacent ring carbons, each ring carbon also bears an H — these two substituents can be on the same face (cis) or opposite faces (trans).