Final answer:
Argon (Ar) has the smallest atomic radius among the given options because it is the furthest to the right in the same period of the periodic table, indicating a stronger nuclear attraction.
Step-by-step explanation:
The element with the smallest atomic radius among Sodium (Na), Chlorine (Cl), Phosphorus (P), and Argon (Ar) can be determined by considering the periodic trends in atomic size. Atomic radius generally decreases across a period from left to right and increases down a group. Given the options are from the same period of the periodic table, Argon (Ar), being a noble gas and the furthest to the right, has the smallest atomic radius because it has more protons than the other elements, leading to a stronger attraction between the electrons and the nucleus. Thus, the correct answer is D. Argon.