Sulfur has a slightly lower first ionisation enthalpy ($1000\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$) than phosphorus ($1012\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$). The best explanation is that

1. A$\mathrm{P}$ has a completely filled $3p$ subshell
2. Bin $\mathrm{S}$ ($3p^4$) the electron removed comes from a paired orbital, and its loss relieves repulsion and leaves the stable half-filled $3p^3$ ion
3. C$\mathrm{S}$ has a lower nuclear charge than $\mathrm{P}$
4. D$\mathrm{S}$ has a smaller atomic radius than $\mathrm{P}$