What is the purpose of a file extension?

Question

It let's an OS know where to look for a file.
It let's an OS know which software Interacts with a file.
It let's an OS know how big a file is.
It let's an OS know which files are safe to delete.

Answer 1

The correct purpose of a file extension is:

It lets an OS know which software interacts with a file.

File extensions help operating systems determine the format of a file and which application is associated with that format for opening or editing the file. For example, a file with a .txt extension is typically opened with a text editor, while a file with a .jpg extension is usually opened with an image viewer.