Which of the following is not the correct increasing order of ionization energy?
Au<Ag<Cu
Let's look at each of the options.
Cl−<Ar<K+
These are isoelectronic species. But look at the number of protons. It is increasing in the given order. So, I.E. also increases.
Au < Ag < Cu
Ionization potential decreases down a group. This is incorrect.
Cs < Rb < K
Ionization potential decreases down a group. This is correct.
K < Ca < Sc
Ionization potential increases across a period. This is correct.