Final answer:
A widow marrying her deceased husband's brother is a cultural practice found in various societies, and it falls under the subject of Social Studies.
Step-by-step explanation:
The subject of this question is Social Studies. Specifically, it relates to the cultural practices and remarriage rules in different societies. In some societies, such as those with polygynous families, the levirate practice is common, where a widow marries one of her deceased husband's brothers to ensure continuity in the family unit. This practice is often seen in Inner Asian Steppe peoples and early Korean ruling families. Another practice is the sororate, which applies when a wife dies and there is a surviving widower, and the deceased wife's lineage provides a replacement female. Ghost marriage is also mentioned, where a marriage is performed between deceased individuals to create an alliance between lineages.