(i) B to Tl
The electric configuration of group 13 elements is $n{s}^2 n{p}^1$. Therefore, the most common oxidation state exhibited by them should be +3. However, it is only boron and aluminium which practically show the +3 oxidation state. The remaining elements, i.e., Ga, In, Tl, show both the +1 and +3 oxidation states. On moving down the group, the +1 state becomes more stable. For example, Tl (+1) is more stable than Tl (+3). This is because
of the inert pair effect. The two electrons present in the s-shell are strongly attracted by the nucleus and do not participate in bonding. This inert pair effect becomes more and more prominent on moving down the group. Hence, Ga (+1) is unstable, In (+1) is fairly stable, and Tl (+1) is very stable.

The stability of the +3 oxidation state decreases on moving down the group.
(ii) C to Pb
The electronic configuration of group 14 elements is $n{s}^{2}n{p}^2$. Therefore, the most common oxidation state exhibited by them should be +4. However, the +2 oxidation state becomes more and more common on moving down the group. C and Si mostly show the +4 state. On moving down the group, the higher oxidation state becomes less stable. This is because of the inert pair effect. Thus, although Ge, Sn, and Pb show both the +2 and + 4 states,
the stability of the lower oxidation state increases and that of the higher oxidation state decreases on moving down the group.

$\stackrel{\mathrm{stability} \mathrm{of} +2 \mathrm{state} \mathrm{increases} }{\to }$
$\mathrm{C} \mathrm{Si} \mathrm{Ge} \mathrm{Sn} \mathrm{Pb}$
$\underset{\mathrm{Stability} \mathrm{of} +4 \mathrm{state} \mathrm{decreases}}{\to }$