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In the following the most stable conformation of n-butane is
A
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B
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C
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D
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Detailed Solution
The conformations of n-butane arise by rotation about the C2–C3 bond and are compared using Newman projections.
Eclipsed conformations have torsional strain; the fully eclipsed form, with the two methyl groups one behind the other, is the least stable.
Staggered conformations are more stable. In the gauche form the two methyl groups are $60^\circ$ apart and repel each other (steric strain).
In the anti (anti-staggered) form the two methyl groups are $180^\circ$ apart, as far from each other as possible, so both torsional and steric strain are minimum.
Order of stability: anti > gauche > eclipsed > fully eclipsed.
Hence the most stable conformation is the staggered Newman projection in which one $CH_3$ points straight up on the front carbon and the other $CH_3$ points straight down on the back carbon.
Eclipsed conformations have torsional strain; the fully eclipsed form, with the two methyl groups one behind the other, is the least stable.
Staggered conformations are more stable. In the gauche form the two methyl groups are $60^\circ$ apart and repel each other (steric strain).
In the anti (anti-staggered) form the two methyl groups are $180^\circ$ apart, as far from each other as possible, so both torsional and steric strain are minimum.
Order of stability: anti > gauche > eclipsed > fully eclipsed.
Hence the most stable conformation is the staggered Newman projection in which one $CH_3$ points straight up on the front carbon and the other $CH_3$ points straight down on the back carbon.