(a) (i) Transition metals show variable oxidation states because of the small difference in energy of the ns and $\left(\mathit{n}-1\right)\mathit{d}$ orbitals. Thus, addition to $\mathit{n}\mathit{s}$ electrons, $\left(\mathit{n}-1\right)\mathit{d}$ electrons also participate in bonding.
(ii) $\mathrm{Zn},\mathrm{Cd}$ and $\mathrm{Hg}$ are not transition elements but are regarded as soft metals as they easily oxidize to +2 oxidation state with low melting point.
(iii) Because ${\mathrm{Mn}}^{2+}$ has ${\mathit{d}}^5$ as a stable configuration whereas ${\mathrm{Cr}}^{3+}$ is more stable due to stable ${\mathit{t}}_{2\mathit{g}}^3.$
(b) Similarity-both are stable in +3 oxidation state/ both show contraction/ irregular electronic configuration (or any other suitable similarity)
Difference-actinoids are radioactive and lanthanoids are not / actinoids show wide range of oxidation states but lanthanoids don't (or any other correct difference).
OR
(a) (i) ${\mathrm{Cr}}^{3+}$ is the most stable in aqueous solution as the tripositive ions with increasing atomic number.
(ii) ${\mathrm{Mn}}^{3+}$ as they have the highest ${\mathrm{E}}^{\circ }$ $\left({\mathrm{M}}^{3+}∕{\mathrm{M}}^{2+}\right)$.
(iii) ${\mathrm{Ti}}^{4+}$ is colourless.
Electronic configuration : $1{\mathit{S}}^{2}2{\mathit{S}}^{2}2{\mathit{p}}^{6}3{\mathit{S}}^{2}3{\mathit{p}}^6$ $4{\mathrm{S}}^{2}3{\mathrm{d}}^{\circ }$
It has completely empty $\mathit{d}$-orbital and there are no electrons for the $\mathit{d}-\mathit{d}$ transition.
(b) (i) $2{\mathrm{MnO}}_4^{-}+16{\mathrm{H}}^{+}+5{\mathrm{S}}^{2-}\to 5\mathrm{S}$ $+2{\mathrm{Mn}}^{2+}+$ $8{\mathrm{H}}_2\mathrm{O}$
(ii) $2{\mathrm{KMnO}}_4\to {\mathrm{K}}_2{\mathrm{MnO}}_4+{\mathrm{MnO}}_2+{\mathrm{O}}_2$