Explanation:

The size of an atom is determined by its atomic radius. Atomic radius is defined as the distance between the nucleus of an atom and its outermost electron.

In the third period of the periodic table, there are 8 elements - Na, Mg, Al, Si, P, S, Cl, and Ar.

Factors affecting atomic radius:

1. Nuclear charge: The greater the nuclear charge, the stronger the attraction between the nucleus and the electrons, and the smaller the atomic radius.

2. Number of electron shells: The more the number of electron shells, the larger the atomic radius.

3. Shielding effect: The more the number of electrons between the nucleus and the outermost electron, the greater the shielding effect, and the larger the atomic radius.

Reasoning:

Out of the 8 elements in the third period, the element having the smallest size is Ar (Argon).

This is because:

1. Ar has the highest nuclear charge (18 protons) among all the elements in the third period. This results in a stronger attraction between the nucleus and the outermost electrons, and hence a smaller atomic radius.

2. All the elements in the third period have the same number of electron shells (3). Hence, the number of electron shells does not affect the atomic radius.

3. Ar has a complete valence shell (8 electrons) and no unpaired electrons. Hence, it experiences the greatest shielding effect among all the elements in the third period, resulting in a larger atomic radius. However, this effect is not sufficient to overcome the strong nuclear charge of Ar, resulting in a smaller atomic radius compared to other elements in the same period.

Therefore, the correct answer is option 'C' - Ar.