Final answer:
Intrasexual selection of mates is not a defining characteristic of eusociality. Eusociality includes reproductive and non-reproductive castes, multi-generational cohabitation, and cooperative care of young. The correct option is B.
Step-by-step explanation:
The characteristic not included in the defining characteristics of eusociality is B. intrasexual selection of mates. Eusociality is defined by the presence of reproductive and non-reproductive castes, multi-generational cohabitation, and cooperative care of the young among animals, particularly insects.
Intrasexual selection, which refers to competition within the same sex for mates, is not a defining characteristic of eusociality, as eusocial societies often have specific castes that handle reproduction, thus reducing or eliminating intrasexual competition for mates. The correct option is B.