1. We are comparing the stability of the dihalides $MX_2$ of group 14 elements: $Si,\ Ge,\ Sn,\ Pb$

2. Group 14 elements show oxidation states $+4$ and $+2$.

   • For the lighter elements like $Si$ and $Ge$, the $+4$ state is more stable.
   • For the heavier elements like $Sn$ and especially $Pb$, the $+2$ state becomes increasingly stable.

3. The reason is the inert pair effect.

   • Down the group, the valence $ns^2$ electrons become less willing to participate in bonding.
   • Hence, the lower oxidation state $+2$ becomes more stable as we move from $Si$ to $Pb$.

4. Therefore, the stability of dihalides $MX_2$ increases down the group: $SiX_2 < GeX_2 < SnX_2 < PbX_2$

5. Now check the options:

   • A: opposite order, incorrect.
   • B: wrong placement of $SiX_2$ and $GeX_2$, incorrect.
   • C: puts $PbX_2$ less stable than $SnX_2$, incorrect.
   • D: $SiX_2 \ll GeX_2 \ll SnX_2 \ll PbX_2$ correct.

Thus, the correct option is D.