Given below are four elements with their atomic numbers:

Element	Atomic Number
$A$	$16$
$B$	$11$
$C$	$3$
$D$	$14$

$(a)$ Identify the elements which belong to the same group of the Modern Periodic Table.
$(b)$ Arrange the given elements in decreasing order of atomic size.
$(c)$ Write the formula of the oxide of $B$.
$(d)$ Which of the above elements is a metalloid?

(A-D) Elements $B$ $(Z=11)$ and $C$ $(Z=3)$ belong to the same group (Group $1$,Alkali metals) because their electronic configurations are $2, 8, 1$ and $2, 1$ respectively,both having $1$ valence electron.
$(b)$ Atomic size decreases across a period and increases down a group. The elements are: $C$ ($Z=3$,Period $2$),$B$ ($Z=11$,Period $3$),$D$ ($Z=14$,Period $3$),$A$ ($Z=16$,Period $3$). Comparing sizes: $B$ $(Z=11)$ > $D$ $(Z=14)$ > $A$ $(Z=16)$ and $C$ is in Period $2$,so it is smaller than $B$. The decreasing order is $B > D > A > C$.
$(c)$ Element $B$ has atomic number $11$,so its valency is $1$. Oxygen has a valency of $2$. Thus,the formula of the oxide is $B_2O$.
$(d)$ Element $D$ (Silicon,$Z=14$) is a metalloid.