Which of the following elements does not belong to the first transition series?

Fill in the blanks:
$(a)$ The $4f$ and $5f$ series elements are known as lanthanoids and actinoids,respectively.
$(b)$ In transition elements,electrons are filled in the $d$-orbitals of the .......... energy levels.

In the series $Sc (Z=21)$ to $Zn (Z=30)$,the enthalpy of atomisation of zinc is the lowest,i.e.,$126 \ kJ \ mol^{-1}$. Why?

The halides of transition elements become more covalent with increasing oxidation state of the metal. Why?

Of the following outer electronic configurations of atoms,which one achieves the highest oxidation state?

Explain on what factors the ionization enthalpies of elements having $d^{n}$ configuration depend upon? Explain the variation in ionization enthalpies of transition elements in the $3d$ series.