Which of the following atoms has the highest first ionization energy?

The highest first ionization energy amongst the given atoms is Sc. Its electronic configuration is $\left(1s{)}^2\right(2s{)}^2(2p{)}^6\mathit{ }(3s{\mathit{)}}^{\mathit{2}}\left(3p{\mathit{)}}^{\mathit{6}}\right(3d{\mathit{)}}^{\mathit{1}}(4s{\mathit{)}}^{\mathit{2}}$. The removal of electron from fully filled 4s orbital requires larger energy as compared to the half-filled s orbital. In general, ionization energy of atoms decreases within a group from top to bottom and it increase in a period from left to right, with the exception at Group 15 to 16.