Which among the following elements has the largest atomic radii?

The electronic configurations of $4$ elements $A, B, C,$ and $D$ are as follows:
$A: 2, 1$
$B: 2, 2$
$C: 2, 8, 2$
$D: 2$
$(a)$ Which of these elements belong to the same group? Name them.
$(b)$ Which of these elements belong to the same period? Name them.
$(c)$ Which of these elements is inert? Name it.

$Na, Mg, Al, Si, P, S, Cl$ and $Ar$ are the elements of the third period.
$(a)$ Which one of them is an inert gas? Why?
$(b)$ Which one of them has a valency of $4$? What kind of bond does it form: ionic or covalent?
$(c)$ Which of them are electropositive? Pick any two.

An element $A$ has an atomic number $16$. To which group and period does it belong?

Write the trend of atomic size and metallic character along a group and a period in the modern periodic table.

State the reason for the following:
$(a)$ The elements of the same group have similar chemical properties.
$(b)$ The elements of the same period have different properties.