The correct order of ionisation enthalpies is F>P>S>B NOTE : On moving along a period ionization enthalpy increases from left to right and decreases from top to bottom in a group. But this trend breaks up in case of atom having fully or half filled stable orbitals. In this case P has a stable half filled electronic configuration hence its ionisation enthalpy is greater in comparison to S. For B(5) electronic configuration is =1s22s22p1 and P(15) electronic configuration is =1s22s22p63s23p3. As P has half filled valence shell so removing electron will be harder compare to B. Hence by checking all options the correct possible order is B<S<P<F