The atom having smallest size in the list below isNa, Si, Cl, Ar(A) Na(B) Cl(C) Si(D) Ar

The size of an atom is determined primarily by its atomic radius, which is influenced by the number of electron shells and the effective nuclear charge experienced by the outermost electrons. In general, as you move from left to right across the periodic table, atomic radius tends to decrease due to increased effective nuclear charge, while moving down a group, atomic radius tends to increase due to the addition of new electron shells.

Given the options you've provided: Na, Si, Cl, Ar

Na (Sodium) is in the third period (row) of the periodic table and has 3 electron shells.
Si (Silicon) is also in the third period and has 3 electron shells.
Cl (Chlorine) is in the third period and has 3 electron shells.
Ar (Argon) is in the third period and has 3 electron shells.
Since all of these elements are in the same period (period 3), their atomic sizes are primarily determined by the number of electron shells, which is the same for all of them. Therefore, there is no significant difference in atomic size among Na, Si, Cl, and Ar within the same period.

So, the answer to your question is: None of the listed atoms (Na, Si, Cl, Ar) has the smallest atomic size in this context because they are all in the same period and have the same number of electron shells.