(a) Because ${\mathrm{Mn}}^{2+}$ is more stable than ${\mathrm{Mn}}^{3+}$ due to half-filled ${\mathrm{d}}^5$ configuration whereas ${\mathrm{Fe}}^{2+}$ becomes unstable after loosing an electron from half filled orbital.
(b) Iron has higher enthalpy of atomization then that of copper because of higher number of unpaired electrons present in iron. $\mathrm{Fe}\left(3{\mathit{d}}^{6}4{\mathit{s}}^2\right)$ possesses and unpaired electron while $\mathrm{Cu}\left(3{\mathit{d}}^{10}4{\mathit{s}}^1\right)$ possesses only 1 unpaired electron.
Number of unpaired electrons $\mathit{\alpha }$ Inter atomic interaction $\mathit{\alpha }$ metallic bonding a entnalpy of atomization.
(c) ${\mathrm{Sc}}^{3+}\left(3{\mathit{d}}^0\right)$ is colourless in aqueous solution due to absence of unpaired electron and no $\mathit{d}-\mathit{d}$ transition takes place. While ${\mathrm{Ti}}^{3+}\left(3{\mathit{d}}^1\right)$ is coloured due to the presence of one unpaired electron and $\mathit{d}-\mathit{d}$ transition takes place which absorbs light in visible region.