Why do the transition elements exhibit higher enthalpies of atomisation?

Question

$\begin{array}{1 1} \text{(a)Because transition elements are electronegative in nature} \\ \text{(b) Because of large number of paired electrons} \\ \text{(c) Because of large number of unpaired electrons.} \\ \text{(d) Because transition elements are ionic in nature} \end{array}$