Topic: Pronoun (সর্বনাম),

Best Answer Explained in Below-:

The correct answer to the question "I don't know __ dress it is." is **D) whose**. This selection is based on the grammatical role that "whose" plays in the sentence.

"Whose" is a possessive pronoun that is used to inquire about ownership or association. In this context, the sentence is asking about the ownership of the dress in question. The speaker is uncertain about to whom the dress belongs, making "whose" the appropriate choice.

In contrast, the other options do not fit grammatically or contextually.

- **A) what** would suggest asking for the type or identity of the dress, which does not align with the possessive context needed here.
- **B) who** refers to a person rather than possession, so it does not apply to the ownership of an object like a dress.
- **C) who's** is a contraction of "who is" or "who has," which again does not address possession.

Therefore, "whose" correctly conveys the intent of the sentence, focusing on the issue of ownership, hence making it the correct choice.