Here's why:
* Abstract nouns represent concepts, ideas, feelings, or qualities that cannot be touched. Examples include *love, happiness, justice, freedom*.
* Concrete nouns represent things that are physical and can be touched. Examples include *book, chair, tree, prison*.
While "prison" is a place of confinement, it is a physical structure that exists in the real world. You can see and touch it. Therefore, it is a concrete noun.
Let me know if you have any other questions!