Ethene, shown, has no geometrical isomers. Which one of the alkenes shown as options can exist as a pair of cis/trans (geometrical) isomers?

Geometrical isomerism requires each doubly bonded carbon to carry two different groups. P, pent-2-ene $\mathrm{CH}{}_3\mathrm{CH}=\mathrm{CHC}{}_2\mathrm{H}{}_5$, has $\mathrm{H}$ and $\mathrm{CH}{}_3$ on one carbon and $\mathrm{H}$ and $\mathrm{C}{}_2\mathrm{H}{}_5$ on the other: cis/trans isomers exist. Q (2-methylbut-2-ene) has two methyls on one carbon, R (but-1-ene) and S (2-methylpropene) have two hydrogens on the terminal carbon; each of these has a carbon with two identical groups, so no geometrical isomerism.