**Smallest in Size:**
The size of an atom or ion is determined by its atomic radius, which is the distance from the nucleus to the outermost electron shell. The atomic radius generally decreases from left to right across a period and increases from top to bottom within a group on the periodic table.

Let's analyze the given options:

(a) Cl- (Chloride ion):
Chloride is an anion formed by gaining one electron. When an atom gains an electron, it becomes larger due to the increased electron-electron repulsion. However, compared to the neutral chlorine atom (Cl), the chloride ion (Cl-) is slightly larger because the added electron increases the electron-electron repulsion and the electron cloud expands. Therefore, Cl- is larger than Cl.

(b) Na (Sodium atom):
Sodium is an alkali metal located in group 1 of the periodic table. Sodium has a single valence electron in its outermost shell. As we move from left to right across a period, the atomic radius generally decreases. Therefore, Na is larger in size compared to Cl-.

(c) Mg (Magnesium atom):
Magnesium is an alkaline earth metal located in group 2 of the periodic table. It has two valence electrons in its outermost shell. The atomic radius of magnesium is larger than sodium because it has an additional electron shell. Therefore, Mg is larger in size compared to Na.

(d) S-2 (Sulfide ion):
Sulfide is an anion formed by gaining two electrons. Similar to chloride ion, when an atom gains electrons, it becomes larger. Therefore, S-2 is larger in size compared to Mg.

In conclusion, the smallest in size among the given options is (a) Cl- (Chloride ion).

Mg+2 Atomic size is inversely proportional to zeff.. So atmoic size is direcrly proportional to negative charge and inversely proportional to positive charge..