Explanation:

The correct answer is option 'B' - whose.

Explanation:

The word "whose" is a pronoun used to show possession or ownership. In this sentence, it is used to indicate that the lady's husband died last year.

Usage of "whose":

The word "whose" is used to relate to a noun or pronoun previously mentioned and show possession or ownership. It is commonly used to introduce a relative clause.

Example:

- This is the lady whose husband died last year.
- The man whose car was stolen reported it to the police.

In both examples, "whose" is used to indicate possession or ownership. It helps to provide additional information about the person or thing being referred to.

Other possible options and their explanations:

- Option 'A' - who: The word "who" is a pronoun used to refer to a person or people. However, in this sentence, we are referring to the lady's husband, not the lady herself. Therefore, option 'A' is incorrect.

- Option 'C' - when: The word "when" is an adverb used to refer to a specific time or period. It does not indicate possession or ownership. Therefore, option 'C' is incorrect.

- Option 'D' - which: The word "which" is a pronoun used to refer to a specific thing or things. However, in this sentence, we are referring to the lady's husband, not a thing. Therefore, option 'D' is incorrect.

Conclusion:

The correct answer is option 'B' - whose, as it correctly shows possession or ownership of the lady's husband.