(a) On the basis of incompletely filled $3\mathit{d}$-orbitals in case of Scandium atom $\left(3{\mathit{d}}^1\right)$ having one electron in its ground state, it is regarded as a transition element. On the other hand, zinc atom having completely filled $\mathit{d}$-orbitals $\left(3{\mathit{d}}^{10}\right)$ in its ground state as well as in its oxidized state, it is not considered as a transition element.
(b) ${\left[\mathrm{Ti}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{+3}$ is coloured because ${\mathrm{Ti}}^{+3}\left(3{\mathrm{d}}^1\right)$ has one unpaired electron which undergoes $\mathit{d}-\mathit{d}$ transition so it is coloured. While ${\left[\mathrm{Sc}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{+3}$ $\left(3{\mathit{d}}^0\right)$ has no unpaired electrons so it will not undergo $\mathit{d}-\mathit{d}$ transition so it is colourless.
(c) Physical and chemical properties of $4\mathit{d}$ and $5\mathit{d}$ series elements are similar because they have same atomic and ionic radii due to the lanthanide contraction. Due to equal atomic radii the size of $\mathrm{Zr}$ and $\mathrm{Hf},\mathrm{Nb}$ and $\mathrm{Ta},\mathrm{Mo}$ and $\mathrm{W}$, etc., the two elements of each pair have the same properties.