The correct option is A arrival of fittest
Darwin's theory of natural selection explains the survival of the fittest, origin of species, competitions, etc. but is unable to explain the arrival of the fittest. Due to the variation some individuals would be better adjusted towards the surroundings than the other. Adaptive modifications are caused through the struggle for existence. According to Darwin, the variations are gradual (continuous) and those which are helpful in the adaptations of an organism towards its surroundings would be passed on the next generation while the others disappear.