The correct order of the first ionization enthalpies is

We know that Ionisation energy increases as we move left to right in a period and decreases as we move down the group.

And the elements given above belong to same period.

Therefore, the correct order of first ionisation enthalpy is $\mathrm{Ti}<\mathrm{Mn}<\mathrm{Ni}<\mathrm{Zn}$.