(s) about the number of stereoisomers (including optical isomers) or optical activity.

(A) is correct: $\mathrm{CH_3CH{=}C(CH_3)CH_2CH_3}$ has no stereocentre and each alkene carbon carries two different groups, so only the $E$ and $Z$ isomers exist. (B) is wrong: 1,2-dimethylcyclopropane has a cis form (meso, with a plane of symmetry) and a trans form (chiral, existing as two enantiomers), so exactly three stereoisomers exist, not two. (C) is correct: penta-2,3-diene has the two terminal carbons each carrying $\mathrm{H}$ and $\mathrm{CH_3}$ in perpendicular planes, which gives axial chirality (a three-dimensional model is non-superposable on its mirror image). (D) is wrong: both the cis and the trans isomers of 1,3-dimethylcyclobutane have a plane of symmetry, so there are only two stereoisomers, both achiral.