Answer: A. Box D, which represents public goods.
Step-by-step explanation:
A public good is referred to as a good that possesses two main characteristics which are non-rivalrous and non-excludable.
This simply means that individuals nobody can be excluded from using a public good and.itsuse by one individual doesn't affect others from using it too.
Since Emma's use of good x does not affect anyone else's use of good x and neither Emma nor anyone else can be prevented from using the good, then it's a public good.