Answer:
The answer to this question an be given as:
The other thing which can be purchased with your $50.
Step-by-step explanation:
In the question the opportunity cost is "The other thing which can be purchased with your $50". Because you have only $ 50 as a birthday gift and you want to choose to buy new wallpaper for your room.If you buy only the wallpaper so you can't buy any other item. To solve this problem you should buy other item. It may be any thing you want. So the answer to this question is "The other thing which can be purchased with your $50".