The sentence structure implies a possessive relationship. A. whose: This relative pronoun is used to show possession and is suitable for connecting the woman to her daughter. The sentence, "The woman whose daughter he married is his mother-in-law," indicates that the daughter belongs to the woman, making "whose" the correct choice. B. which: "Which" is typically used for things rather than people and may not convey the intended meaning in this context. C. her: The sentence structure requires a relative pronoun, and "her" is a pronoun, not a relative pronoun like "whose." D. whom: "Whom" is used as an object pronoun, and in this context, a possessive form is needed to show the relationship between the woman and her daughter. Therefore, option A, "whose," is the most appropriate choice for completing the sentence and indicating the possessive relationship between the woman and her daughter.