(a) The two consequences of lanthanide contraction are:
(i) Due to close similarity in electronic configuration and ionic radii, the lanthanides have identical chemical properties which makes their separation difficult.
(ii) Due to lanthanide contraction, the size of ${\mathrm{Ln}}^{3+}$ ions decrease regularly with increase in atomic number. According to Fajan's rule, decrease in size of ${\mathrm{Ln}}^{3+}$ ions increase the covalent character and decrease the basic character between ${\mathrm{Ln}}^3+$ and ${\mathrm{OH}}^{-}$ion in $\mathrm{Ln}{\left(\mathrm{OH}\right)}_3$. Since the order of size of ${\mathrm{Ln}}^{3+}$ ions are ${\mathrm{La}}^{3+}>{\mathrm{Ce}}^{3+}...>{\mathrm{Lu}}^{3+}$.
(b) The element which shows largest number of oxidation states in $3\mathit{d}$ series is Manganese (Mn). It shows the following oxidation states $+2,+3$, $+4,+5,+6$ and +7 . It is because it contains the maximum no. of unpaired ${\mathit{e}}^{-}\mathit{s}$ in the outermost shell, i.e., $3{\mathit{d}}^{9}4{\mathrm{}\mathit{s}}^2$.