The first ionization energy corresponds to the energy required to ionize an atom, that is, to separate an electron from its ground state.
This energy depends on three factors:
1) Of the energy levels that the atom has. Since the fewer energy levels, the more electrons will be attracted to the nucleus of the atom. In this case, chlorine and argon have the same number of energy levels, that is, they both have three energy levels.
2) It also depends on the number of electrons in the last energy level. Chlorine is in period 7 of the periodic table and argon in period 8. That is, chlorine has 7 valence electrons and argon 8 valence electrons. Electrons repel each other, so more electrons means less ionization energy.
3) The nuclear charge (given by the atomic number, Z), since the higher Z is, the greater the force of attraction exerted by the nucleus on the electrons and therefore the more difficult it is to extract them. The atomic number of chlorine is 17 and the atomic number of argon is 18.
Now, the ionization energy of argon is higher than that of chlorine. But having more electrons is not the reason why it has a higher ionization energy as explained above. On the contrary, since argon has a higher atomic number, it implies that the effective charge of the nucleus is greater and the attraction of electrons to the nucleus increases. increasing the ionization energy.
Asnwer: D Ar, because of its higher effective nuclear charge