A simple operating system supports only a single directory but allows it to have arbitrarily many files with arbitrarily long file names. Can something approximating a hierarchical file system be simulated? How?

Yes

Explanation:

Yes, something approximating a hierarchical file system be simulated. This is done by assigning to each file name the name of the directory it is located in.

For example if the directory is UserStudentsLindaPublic, the name of the file can be .

Also the file name can be assigned to look like the file path in the hierarchical file system. Example is /user/document/filename