Which version of the license key library should I use?

This depends on which version of iText you are using:

If you're using iText Suite version 7.2 (or newer) you'll need to use version 4.x.y of the license key library
If you're using iText Suite version 7.1.17 (or older) or iText 5.5.13 (or newer) you'll need to use version 3.x.y of the license key library
If you're using iText 5.5.12 (or earlier), you'll need to use version 1.x.y of the license key library

Please use the latest version within the appropriate major version to have the latest features and bug fixes available.