Consider all isomers of molecular formula C5H10.

(A) Correct. The cyclic structural isomers are cyclopentane, methylcyclobutane, ethylcyclopropane, $1,1$-dimethylcyclopropane and $1,2$-dimethylcyclopropane. (B) Incorrect. Only $1,2$-dimethylcyclopropane has stereoisomers: the $cis$ form is meso (one isomer) and the $trans$ form exists as a pair of enantiomers, so the other four give $4$ and this one gives $3$, a total of $7$, not $6$. (C) Correct. Only the two $trans$ enantiomers lack an internal mirror plane; $cis$ is meso and the rest are achiral. (D) Incorrect. The acyclic constitutional alkenes are $1$-pentene, $2$-pentene, $2$-methyl-$1$-butene, $3$-methyl-$1$-butene and $2$-methyl-$2$-butene, which is $5$; only $2$-pentene shows geometric isomerism, so the total is $6$, not $7$.