Both crabs and lobsters are members of Pleocyemata. Both possess a tail but in crab, it is very small and not used. This is an example of

Convergent evolution refers to the development of similar structure in two distant organisms as a response to adaptation to the same environment; both crab and lobster belong to the same order. Divergent evolution in closely related organisms leads to the formation of the homolgous structure that performs same function. The poorly developed organs of recent forms of organisms which were fully developed and functional in related ancestral forms are referred to as vestigial organs. Genetic fitness is related to the ability of reproduction. Thus, the correct answer is option B.