The increasing order of the atomic radius of Si, S, Na, Mg, Al is

The increasing order of the atomic radius of $\mathrm{Si}, \mathrm{S}, \mathrm{Na}, \mathrm{Mg}, \mathrm{Al}$ is $\mathrm{S}<\mathrm{Si}<\mathrm{Al}<\mathrm{Mg}<\mathrm{Na}$.

All of these elements belong to the third period of the periodic table. On moving from left to right across the period, the atomic radius decreases.

Hence, the required answer is A.