Final answer:
The chair conformations of trans-1,3-dimethylcyclohexane feature one axial and one equatorial methyl group because of the trans configuration which makes it impossible for both methyl groups to be equatorial at the same time.
Step-by-step explanation:
The correct description of the chair conformations of trans-1,3-dimethylcyclohexane is that one methyl group is axial, and the other is equatorial (option b). This is because in trans-1,3-dimethylcyclohexane, the two methyl groups are on opposite sides of the ring. When the molecule is in one chair conformation, one of the methyl groups will necessarily be in an axial position, while the other will be in an equatorial position. Flipping the chair conformation will reverse these orientations, but still one will be axial and the other equatorial.
Steric repulsion and 1,3-diaxial interactions typically dictate that the more stable conformation is the one with the larger groups in the equatorial positions, as this reduces steric strain. However, in the case of trans-1,3-dimethylcyclohexane, because of the fixed trans configuration, we cannot have both methyl groups occupying equatorial positions simultaneously.