Final answer:
The incest taboo is a universal cultural norm that forbids sexual relations between close family members, helping to maintain family and kinship structures by setting clear boundaries. The correct answer is option: a. limits sexual competition within families.
Step-by-step explanation:
The incest taboo is a universal prohibition against sexual relations between close kin, particularly between parents and their offspring, and sometimes extends to other relatives deemed too close for sexual relationships. It varies from culture to culture which relatives are included in this prohibition.
The presence of an incest taboo across all societies contributes to the structuring of families and broader kinship networks by distinctly separating the types of relationships one can have with different categories of relatives. It helps to limit sexual competition within families, clarify people's rights and obligations towards each other, and maintain the kinship and societal structure.