Which of the following elements exhibits both positive and negative oxidation states?

In view of the signs of $\Delta_{r} G^{\circ}$ for the following reactions:
$PbO_2 + Pb \longrightarrow 2 PbO, \Delta_{r} G^{\circ} < 0$
$SnO_2 + Sn \longrightarrow 2 SnO, \Delta_{r} G^{\circ} > 0$
Which oxidation states are more characteristic for lead and tin?

Which of the properties remains unchanged on descending a group in the periodic table?

Variable valency is shown by

If the valency of $A$ is $3$ and $B$ is $2$,then the formula of the compound formed is:

Among the following,the number of elements showing only one non-zero oxidation state is $O, Cl, F, N, P, Sn, Tl, Na, Ti$.