$\operatorname{Sc}(Z=21)$ is a transition element but $\mathrm{Zn}(Z=30)$ is not because\

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(b) : Sc $(Z=21)$ has incompletely filled 3d-orbitals in its ground state ( $3 d^{1}$ ), it is considered as a transition element but $\mathrm{Zn}(Z=30)$ has completely filled $d$-orbitals $\left(3 d^{10}\right)$ in its ground state and its common oxidation state ( +2 ), thus, it is not considered as a transition element.