The anti-conformation is the most stable conformation of n-butane. In this, the bulky methyl groups are as far apart as possible thereby keeping steric repulsion at a minimum.

Alkenes with double bonds cannot undergo free rotation and can have different geometrical shapes with two different groups on each end of the double bond.

$$ \therefore $$ C₂ - sp, C₃ - sp³, C₅ - sp² and C₆ - sp³