Final answer:
The outermost electron experiences the greatest effective nuclear charge in sodium (Na).
Step-by-step explanation:
The atom in which the outermost electron experiences the greatest effective nuclear charge is d) Na, which is sodium. Sodium has 11 protons in its nucleus, giving it a positive charge of +11. The outermost electron in sodium is in the 3s orbital, and it experiences a strong attractive force from the nucleus due to the high nuclear charge. This results in the outermost electron feeling the greatest effective nuclear charge in sodium.