The stem carries a drawn cyclohexane structure; the four options are text and are reproduced in full above.

A substituted cyclohexane sits in a chair conformation and flips between two chairs. Every substituent that is axial in one chair becomes equatorial in the other, so the stable conformer is the one that places the bulkiest groups equatorial.

The reason axial is disfavoured is 1,3-diaxial strain: an axial substituent points straight up alongside the axial hydrogens on carbons 3 and 5, and the resulting steric clash costs energy. That cost is measured by the A value, and it rises sharply with bulk:

-OH about 0.9 kcal mol^-1, -CH₃ about 1.7 kcal mol^-1, -iPr about 2.2 kcal mol^-1 (and tert-butyl about 4.9, large enough to lock a ring completely).

So isopropyl has the strongest preference for the equatorial position, methyl the next, and hydroxyl the weakest — the OH group is small and can also relieve strain by hydrogen bonding.

The best conformer is the one with all three groups equatorial, provided the stereochemistry drawn permits it. Whether it does depends on the relative configuration: substituents on adjacent carbons can both be equatorial only if they are trans, while on carbons 1 and 3 they must be cis. When the configuration allows all three to go equatorial simultaneously, that arrangement is unambiguously the most stable, since it removes every 1,3-diaxial interaction at once.

Per the official final answer key the answer is option (A) — iPr, CH₃ and OH all equatorial.