The incorrect statement is

 Ionisation enthalpy of alkali metals decreases down the group from Li to Cs. This is because of increase in size which outweighs the increasing nuclear charge, and the outermost electron is very well screened from the nuclear charge.

$\mathrm{Li}>\mathrm{Na}>\mathrm{K}$

Due to stable electronic configuration noble gases exhibit very high ionisation enthalpy. However, it decreases down the group with increase in atomic size.

$\mathrm{He}>\mathrm{Ne}>\mathrm{Ar}>\mathrm{Kr}>\mathrm{Xe}>\mathrm{Rn}$

Alkaline earths have higher ionization enthalpy values than alkali metals. This is because the atomic size of alkaline earths is smaller than that of alkali metals.

$\mathrm{Sr}>\mathrm{Rb}$

Because of completely filled configuration, zinc is stable and requires higher ionization energy whereas gallium is yet to fill its p-orbitals, so it has lower ionization energy compared to zinc.

$\mathrm{Zn}>\mathrm{Ga}$