Why do the d-block elements exhibits a large number of oxidation states than f - block elements

Question

$\begin{array}{1 1} \text{(a)This is because the difference of energy between (n-1) d and ns Orbitals is equal } \\ \text{(b) This is because the difference of energy between (n-1) d and ns Orbitals is very small} \\ \text{(c) This is because the difference of energy between (n-1) d and ns Orbitals is very large} \\ \text{(d) This is because the difference of energy between (n-1) d and ns Orbitals is zero } \end{array}$