The primary difference is the security access of the user account calling the script. For examples B and C, the security context is executed as the web server user account. For A, it's the one belonging to the crontab owner. If you use option A with a dedicated account to this script, then you've got finer grain access control, because the webserver will likely have other security constraint requirements/extended access.
Secondly, since you are hosting this script on a webserver at http://example.com, then you've opened up significantly more access to trigger the script. If it's desirable to provide more convenient access to run the script, then this is good. Otherwise it's not only less secure, but it's not as efficient since you also have the memory/processor overhead of running an actively listening web server if it's not already being used for other purposes.