$ A=\left\{2,4,6\right\}$,
$B=\left\{2,6\right\}$,
$C=\left\{4,6\right\}$,
$D=\left\{4,6,8\right\}$
set $A$ is said to be subset of $B$ if and only if every elements of $A$ is also an elements of $B$. That is:
$A\subseteq B=\forall x(x\in A\to x\in B)$
So
1) Every elements of B is also an elements of A so $B\subseteq A$
2) Every elements of C is also elements of A so $C\subseteq A$
3) Every elements of C is also an elements of D so $C\subseteq D$