Final answer:
The true statement regarding marriage customs is that polygamy often exists alongside monogamy and is not the sole marriage practice in polygamous societies.
Step-by-step explanation:
The concern over various marriage customs can be addressed by looking at the provided statements. After reviewing the anthropological context, it can be concluded that the correct statement is: b. Where polygamy exists, it is found as an alternative to monogamy rather than the only type of marriage practiced. Polygamy is the socially sanctioned union of more than two adults and, while it is less common than monogamy, it often coexists with monogamous practices within a society. Evidence suggests that same-sex marriage does exist outside Western societies; there are societies with different forms and levels of incest taboos, and though most marriages involve living people, some cultures have ceremonial forms of marriage where one or both participants are not living. Moreover, fraternal polyandry can serve economic and social functions beyond addressing a shortage of women.