In the following the most stable conformation of n-butane is

Key Idea The conformation in which the heavier groups are present at maximum possible distances, so that the forces of repulsion get weak, is more stable. Among the given conformations of n-butane, the conformation shown in option (b) ie, anti conformation is most stable as in it the bulkier group (ie, $\mathrm{CH}_3$ group) are present at maximum possible distance.