Final answer:
Oxygen (O) has the largest shielding effect among the listed elements. The correct answer is option: a. O
Step-by-step explanation:
The element that has the largest shielding effect is Oxygen (O).
Shielding is determined by the probability of another electron being between the electron of interest and the nucleus, as well as by the electron-electron repulsions the electron of interest encounters.
Core electrons are adept at shielding, while electrons in the same valence shell do not block the nuclear attraction experienced by each other as efficiently.
Oxygen has a larger shielding effect compared to other elements listed in the question, such as Scandium or Selenium.