(a)
(i) Due to ability pf oxygen to form multiple bond pπ−pπ bonds.
(ii)Due to lanthnoid contraction, Hf (Z=72) has size similar to that of Zr (Z=40). Hence, their properties are similar.
(iii) Transition metals act as catalysts due to their ability to adopt multiple oxidation states and to form complexes.
(i)2MnO2+4KOH+O2△−→ 2K2MnO4+2H2O(ii)Cr2O2−7+14H++6I−−−−−−−−−−−−→ 2Cr3++7H2O+3I2