Final answer:
The meso form of 1,3-dimethylcyclohexane is cis-1,3-dimethylcyclohexane because it has an internal plane of symmetry when the methyl groups are on the same side of the ring, which makes it superimposable on its mirror image.
Step-by-step explanation:
The student has asked which one of the stereoisomers of 1,3-dimethylcyclohexane can be considered a meso form. The concept of meso compounds arises in the field of stereoisomerism, which is part of organic chemistry. These are molecules that have multiple stereocenters and an internal plane of symmetry, causing them to be superimposable on their mirror image, and therefore they are achiral.
Considering the options provided, the meso form is the one that must have an internal plane of symmetry. For 1,3-dimethylcyclohexane, the cis-1,3-dimethylcyclohexane is the meso form because when both methyl groups are on the same side of the ring (cis), and the molecule is in its most stable chair conformation, it will have a plane of symmetry cutting through the center of the cyclohexane ring, bisecting both methyl groups. Hence, the correct answer to the question is: a. cis-1,3-dimethylcyclohexane.