Final answer:
The electrons in the 2s subshell are much closer to the nucleus in Ar than in He due to the large nuclear charge.
Step-by-step explanation:
The correct answer is C. nuclear charge.
In Ar, the electrons in the 2s subshell are much closer to the nucleus compared to He because of the large nuclear charge in Ar. The nuclear charge in an atom is determined by the number of protons in the nucleus. Ar has a higher atomic number (number of protons) compared to He, which means it has more positive charges in the nucleus. This higher nuclear charge attracts the electrons in the 2s subshell more strongly, pulling them closer to the nucleus.