(a) Why is chemistry of actinoids complicated as compared to lanthanoids?

(b) Complete the following reaction and justify that it is a disproportionation reaction:

$$3 \mathrm{MnO}_4^{2-}+4 \mathrm{H}^{+} \rightarrow$$________ $$+$$ _________ $$+$$ $$\mathrm{H}_2 \mathrm{O}$$.

(c) The given graph shows the trends in melting points of transition metals:

Explain the reason why $$\mathrm{Cr}$$ has highest melting point and manganese $$(\mathrm{Mn})$$ a lower melting point.