Which directory permission is best for publicly readable directories on a Unix web server?

The choice of permissions represented by rwxrwxr-x (775) is appropriate for publicly readable directories on a Unix web server. This permission setting ensures that the owner and group members have full read, write, and execute permissions, while all other users have read and execute permissions only.

Allowing read permissions for all users means that anyone accessing the web server can view the files within the directory, which is essential for a web server scenario where users need to access web content. The execute permission for all users is also crucial because it allows them to navigate through the directory.

In contrast, the other choices either provide excessive permissions or insufficient access for public users. For instance, rwxrwxrwx (777) grants write access to everyone, which poses a significant security risk by allowing anyone to modify the content of the directory. Options with lesser permissions such as rwxrwx-w- (772) or rwxrwx-x (771) limit access for public users, preventing them from reading or executing files in the directory, which would be counterproductive for the intended purpose of making content accessible on the web. Thus, rwxrwxr-x (775) strikes a balanced approach for directory permissions on a Unix web server.