Which periodic trend is responsible for the observation that the maximum oxidation state of the transition-metal elements peaks near groups 7B and 8B?

(a) The number of valence electrons reaches a maximum at group 8B.

(b) The effective nuclear charge increases on moving left across each period.

(c) The radii of the transition-metal elements reaches a minimum for group 8B and as the size of the atoms decreases it becomes easier to remove electrons.