(a) Transition metals have high enthalpies of atomisation due to strong metallic bonding and additional covelent bonding. Metallic bonding is due to their smaller size while covalent bonding is due to d-d overlapping.
(b) In lower oxidation states, transition metals behave like metals and metal oxides are basic in nature. Thus, in lower oxidation states, transition metal oxides are basic.
As the oxidation state increases, its metallic character decreases due to decrease in size, thus, it becomes less metallic or more non-metallic. Oxides of a non-metal may be acidic or neutral. Thus, in higher oxidation states, transition metal oxides are amphoteric or acidic.
(c) In the presence of complexing agents, cobalt gets oxidised from + 2 to + 3 state because it provides energy to remove an electron from Co2+. Moreover, Co (III) is more stable than Co(II).
