Elements Na, Mg, Al, and Si are in Period 3. Across a period from left to right: - Atomic number rises. - Effective nuclear charge increases. - Electrons enter the same shell, strengthening the nucleus's pull on outer electrons. Consequently, atomic radius diminishes across a period. Atomic radius ranking: \[ \text{Na} > \text{Mg} > \text{Al} > \text{Si} \] Answer: Si exhibits the smallest atomic radius among these elements. Option (4) is the correct choice.