Here's why:
* Abstract nouns refer to concepts, qualities, or ideas that cannot be touched or seen. Amusement is a feeling or state of being entertained and lighthearted. You can't physically hold or see amusement.
* Concrete nouns refer to physical things you can touch and see. For example, a "clown" could be a source of amusement, but "clown" itself is a concrete noun.
Therefore, "amusement" is an abstract noun because it represents a concept, not a physical thing.