Which of the following is most stable ?

IV A group elements
Common oxidation state is : "+4"
C shows :- +4 -4
Si:- +4 [Stable]
 

$$\begin{gathered} \mathrm{Ge},\mathrm{Sn}:+2,\underset{\mathrm{Stable}}{+4} \\ \mathrm{Pb}: \underset{\mathrm{Stable}}{+2},+4 \end{gathered}$$

→ For Ge,Sn +4 is stable oxidation state hence in +2 oxidation state they acts as reducing agents
→ Where as for Pb +2 oxidation state is state [due to inert pair effect] hence in +4 oxidation state Pb acts as oxidising agent
→ Higher oxidation state is due to involvement of ns and np electron
i.e., 

Lower oxidation state is due to involvement of only np electron
i.e., ns² np⁽²⁾ ⇒ +2
Higher oxidation state stability decreases from top to bottom
 

Lower oxidation state stability increases from top to bottom
Ge⁺² < Sn⁺² < Pb⁺² Stability order