Cyclopropane, shown, has no stereoisomers. Treat all rings as planar and consider configuration only. Which one of the ring compounds shown as options is chiral?

P is cis-1,2-dimethylcyclopropane: both methyls on the same face, and the plane perpendicular to the ring through C3 and the midpoint of C1-C2 is a mirror plane, so it is meso. Q is trans-1,2-dimethylcyclopropane: the two stereocentres are $(R,R)$ (or $(S,S)$ in its mirror image) and there is no symmetry plane, so Q is chiral and exists as a pair of enantiomers. R is trans-1,3-dimethylcyclobutane, which has a mirror plane through C1 and C3 (C1 and C3 are not stereocentres because the two ring paths between them are identical). S, 1-bromo-1-methylcyclopropane, has a mirror plane through C1 and the midpoint of C2-C3.