(a) Phosphourous `(P_4)` and vanadium `(V)` both exhibit variable oxidation states but due to different reason.
(b) Name the transition metal which does not show variable O.S.

Explanation

(a). In phosphorous `(P_4)` it is due to inert pair effect whereas in vanadium (V) it is due to the participation of d-orbital electrons.
(b). Scandium (Sc) `(Z=21),3d^(1)4s^2`
`Sc^(2+):3d^(1)implies` Highly unstable, does not exist.
`Sc^(3+):3d^(0)implies` Noble gas configuration, stable and exist. Therefore Sc shows only `+3O.S.`