Which of the following disubstituted cyclohexanes is chiral? (Assume rapid chair-chair interconversion, so that only the configuration matters.)

A ring compound is achiral when its time-averaged (planar) structure has a mirror plane. cis-1,2-Dimethylcyclohexane is meso (mirror plane between C1 and C2), and both 1,4-isomers have a mirror plane through C1 and C4. In trans-1,2-dimethylcyclohexane the two methyls lie on opposite faces, no mirror plane exists, and the molecule exists as a (1R,2R)/(1S,2S) pair. So only the trans-1,2-isomer is chiral.