The correct option is D Dipole moment of anti conformer is zero
Non-polar compounds have zero dipole moment.
In gauche conformer of n-Butane the methyl group are in similar direction.
In anti conformers of n-Butane the methyl groups are in opposite direction.
Generally, anti conformers have zero dipole moment and gauche conformer has non-zero dipole moment.
But in n-Butane, both gauche and anti form has zero dipole moment because the electronegativity difference is zero in case of alkanes so bond polarity is almost zero. Hence, alkanes are non-polar.
Therefore, all conformers of n-butane will have zero dipole moment and are non-polar.