Final answer:
A backpack left in a library and taken by someone else is likely considered lost property, and taking it without the intent to return can be classified as theft.
Step-by-step explanation:
A backpack left in the library and later taken by another student is likely considered lost property. In legal terms, lost property refers to items that have been unintentionally left by their owner. If an item like a backpack is found and the finder knows the identity of the owner or has a means to identify and locate the owner, the correct action is to return it.
Taking someone else's property without the intent to return it can be classified as theft or larceny, depending on the local jurisdiction's laws. When a person finds lost property, they typically have certain responsibilities, such as reporting or turning it over to authorities, especially if they cannot directly return it to the rightful owner.