Choose the word which is different from the rest.

The answer is D. Curtain.

A mattress is a large, soft pad on which a person sleeps. A pillow is a soft, usually square or rectangular cushion used to support the head or neck while resting or sleeping. A bedsheet is a piece of cloth used to cover a bed. A curtain is a piece of cloth or other material that hangs over a window or door to keep out light or to provide privacy.

Curtains are different from the other options because they are not used to sleep on. Mattresses, pillows, and bedsheets are all used to sleep on, while curtains are used to keep out light or to provide privacy.