You have to first go through the paragraph carefully and then select the answer of the related questions from the option given:
Transition elements show varibale oxidation state. This is because their valence electrons are found in two different orbitals i.e. ns and (n-1)d. Up to +2 oxidation state ns electrons are involved but in higher oxidation states electrons of (n-1)d subshell are involved.
The cause of variable valency among transition elements is
(A) They all exist in more than one oxidation states
(B) They all form complex compounds
(C) The valence electrons of them are found in two different subshells
(D) They all have paired electrons
