In the more stable chair conformation of tert-butylcyclohexane, the tert-butyl group occupies:

AAn axial position, to avoid gauche interactions with ring carbons
BAn equatorial position, because it increases angle strain
CAn axial position, because it is less crowded there
DAn equatorial position, to avoid 1,3-diaxial interactions

Answer: D

An axial tert-butyl group would suffer severe syn-diaxial repulsion with the two axial hydrogens on the same face (C3 and C5). The equatorial position points away from the ring and avoids this, so the equatorial conformer is far more stable (A-value above 20 kJ/mol).

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