Which of the following has highest second ionization energy?

Electronic configuration for chromium is : { }_{24} $\mathrm{Cr} \rightarrow$1 s² 2 s² 2 p⁶ 3 s² 3 p⁶ 3 d⁵ 4 s¹ After ionisation of $4 s$ electron the next electron which will be ionised is from $3 d$ orbital. As $3 d^5$ electronic configuration, is stable, hence, the second ionisation of $\mathrm{Cr}$ will require much greater ionisation energy than the expected one.