Arrange the elements N, P, O

(i) S< P< O< N is the increasing order of the first ionisation enthalpy.

As we go down the group the ionisation enthalpy decreases and as we go along the period then it increases but in case of oxygen and nitrogen due to half-filled stability of 2 p orbitals of nitrogen it has a higher ionisation enthalpy than oxygen.

(ii) P

As we go down the group non-metallic character decreases as the effective nuclear charge on the outermost shell decreases which helps to gain an electron. As we move along the period then effective nuclear charge increases which increase the non-metallic character.