The correct answer is D. some.
The word “some” is used to refer to an indefinite number or quantity of something. In this case, the speaker is asking the listener to pick a dress, but they are not specifying how many dresses the listener should pick. The listener could pick one dress, two dresses, or even more than two dresses.
The word “much” is used to refer to a large amount of something. In this case, the speaker would be asking the listener to pick a lot of dresses. However, the speaker is not asking the listener to pick a lot of dresses, so the word “much” is not the correct answer.
The word “more” is used to refer to a greater amount of something than something else. In this case, the speaker would be asking the listener to pick more dresses than someone else. However, the speaker is not asking the listener to pick more dresses than someone else, so the word “more” is not the correct answer.
The word “either” is used to refer to one of two things. In this case, the speaker would be asking the listener to pick either a dress or something else. However, the speaker is only asking the listener to pick a dress, so the word “either” is not the correct answer.