A2 Chemistry Transition metals and ionization energy Multiple Choice Question Watch
If you consider how C would ionise, it'd firstly lose its two s-subshell electrons (the first two ionisation energies), and then it'd lose its five d-subshell electrons (the five ionisation energies after that)
When the d-subshell electrons are ionised, electrons are then ionised from the p-subshell, which is significantly closer to the nucleus and experiences less shielding, and so more energy is required to remove electrons from it
Compare this to the other three choices...
A is representative of a group 1 metal
B is representative of a group 2 metal
D is representative of some ionisations which haven't started with the first