A single pure stereoisomer is called a meso compound if it has stereocentres but is optically inactive because of an internal plane of symmetry.…

For each compound the question is whether a single stereoisomer is superimposable on its mirror image while still containing stereocentres. In cis-1,2-dimethylcyclopentane the two methyls lie on the same face and a mirror plane perpendicular to the ring through the midpoint of C1–C2 maps the molecule onto itself, so it is meso; (A) is correct. The trans-1,2 isomer has $C_2$ symmetry only, so it is chiral and exists as an enantiomeric pair; (B) is wrong. In 1,3-dimethylcyclohexane it is the cis isomer that is meso, while the trans isomer is chiral; (C) is wrong. In 1,4-dimethylcyclohexane C1 and C4 carry two identical ring paths and are not stereocentres at all; the trans isomer is achiral but not meso; (D) is wrong. Only (A) is correct.