The correct option is B
In chair conformation of cyclohexane when a substitutent present at axial position, the conformer will be less stable because it has 1, 3 diaxial interaction which is a steric interaction of axial group.
When a substituent presnt at equatorial position, the conformer is stable because the substituent has more room and fewer steric interactions when it is at equatorial position.
In (a), both the substituents are at axial position so it is the least stable conformer.
In (c), one methyl group is at axial position and one methyl is at equatorial. Thus, it is also less stable conformer.
In (d) also, one methyl group is at axial position and one methyl is at equatorial. Thus, it is also less stable conformer.
In (b), both the methyl group are at equatorial position so it is most are stable conformer.