Explanation:

- Correct answer: Option 1, Sororate

- Sororate refers to the custom where a man marries his wife’s sister, usually after the wife's death.

- The term for a woman marrying her husband’s brother is *Levirate*, not sororate or the other options.

- Option 2, Natal: This does not relate to marriage customs, it generally means 'relating to birth.'

- Option 3, Companionate: Companionate marriage means marriage based on mutual friendship or love, not a specific kinship rule.

- Option 4, Polygyny: This is a form of marriage where a man has multiple wives at the same time.

- Hence, none of the given options specifically mean a marriage between a woman and her husband’s brother (which should be levirate), but Sororate is at least related to kin marriage customs.