Cyclohexane exist as two chair conformations in rapid equilibrium at room temperature.
Each carbon atom on a cyclohexane ring has one axial and one equatorial bydrogen. Rind-flipping converts axial H's to equatorial H's and vice-versa

In substituted cyclohexane, groups larger than hydrogen ar more stable in the equatorial position.
The cis isomers has two groups on the same side of the ring, eithere both up or both down.
The trans isomer has two groups on opposite side of the ring one up and one down.
Which of the following is most stable conformer of 1,2-dichlorocyclohexane?