Question

Why potential energy is zero at 90 degree in a dipole in electric field?

Answer 1

Because it is consistent with our choice of zero of the potential at infinity.

When the dipole moment is perpendicular to electric field (uniform for simplicity) both the charges are at an equipotential surface. Let the potential at this surface be V (again with our choice of zero potential at infinity). The work done in bringing +q charge from infinity to point A is +qV and that in bringing -q charge is -qV. The total work done is zero. It is this zero work because of which the potential energy of the dipole in this orientation