Which of the following best describes how the effective nuclear charge experienced by a valence electron changes when moving from sodium (Na) to chlorine (Cl) across Period 3?
a) It remains approximately constant because each added proton is completely counteracted by an added electron in the same shell.
b) It decreases because additional valence electrons provide significant shielding that outweighs the increased nuclear charge.
c) It increases because additional protons are added while shielding by inner electrons remains nearly constant.
d) It fluctuates because of alternating filling of the 3s and 3p orbitals across the period.