What type of marriage involves having multiple spouses over a lifetime, but only one spouse at a time?

The correct answer is serial monogamy, which refers to a pattern of having one spouse at a time but engaging in multiple marriages throughout a lifetime. This practice can involve marrying, divorcing, and then marrying again, effectively having a series of monogamous relationships. Each marriage is exclusive while it lasts, but individuals may move from one spouse to another over time.

In contrast, polygamy generally refers to the practice of having multiple spouses simultaneously and does not fit the description of having only one spouse at a time. Monogamy simply means being married to one person at a time without the implication of moving on to other spouses later. Polyandry specifically refers to a woman having multiple husbands simultaneously, which again does not align with the concept of serial monogamy. Thus, serial monogamy is the only option that accurately describes the situation of having multiple spouses over a lifetime while maintaining a single marital relationship at any given moment.