Out of all the four given metallic oxides $$Cr{O_2}$$  is attracted by magnetic field very strongly. The effect persists even when the magnetic field is removed. Thus $$Cr{O_2}$$  is metallic and ferromagnetic in nature.

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Stable oxidation state of $$Mn$$  in alkaline medium is$${{ + 6}}{{.}}$$ 
So, $$Mn{O_2}$$  is oxidised to $${K_2}Mn{O_4}$$  (purple green) by atmospheric oxygen in $$KOH$$  medium.
$$2Mn{O_2} + 4KOH + {O_2} \to \mathop {2{K_2}Mn{O_4}}\limits_{\left( {{\text{Purple}}} \right)} + 2{H_2}O$$

$$Hg$$  is the last element of third transition series.

$${}_{21}Sc = 1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}3{d^1},4{s^2}$$
$$So,\,\,S{c^{3 + }} = 1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}$$
( It is colourless due to the absence of unpaired electrons in $$d$$ sub-shell )
$${}_{26}Fe = 1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}3{d^6},4{s^2}$$
$$F{e^{2 + }} = 1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}3{d^6}$$

( It is coloured due to the presence of four unpaired electrons in $$d$$ sub-shell )
$${}_{22}Ti = 1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}3{d^2},4{s^2}$$
$$T{i^{3 + }} = 1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}3{d^1}$$

( It is coloured due to the presence of an unpaired electron in $$d$$ sub-shell )
$${}_{25}Mn = 1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}3{d^5},4{s^2}$$
$$M{n^{2 + }} = 1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}3{d^5}$$

( It is coloured due to the presence of five unpaired electrons in $$d$$ sub-shell )

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Promethium $$(Pm)$$  is the element which is prepared only by synthetic methods. It is not present in nature. It is the only synthetic radioactive lanthanoid.

The electronic configuration of $$Sc$$  is
$$1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}3{d^1},4{s^2}$$
and configuration of $$S{c^{3 + }}$$  is
$$1{s^2},2{s^2}2{p^6},3{s^2}3{p^6}$$
So, $$S{c^{3 + }}$$  is colourless due to absence of electrons in $$d$$-orbital.

In the Wacker process, the oxidation of ethyne to ethanal is catalysed by $$PdC{l_2}.$$

In $$C{u^ + }\left[ {Ar} \right]3{d^{10}}$$   there is no unpaired electron, $$C{u^{2 + }}\left[ {Ar} \right]3{d^9}$$   contains one unpaired electron hence coloured.