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encyclo - 12:13 am on May 29, 2007 (gmt 0)
None of them. ;) Mac OSX was built on a platform that evolved from FreeBSD [freebsd.org], which is not Linux at all, but like Linux it is a Unix-clone. FreeBSD has a more liberal license than Linux. FreeBSD is a great server OS, harder to set up than the Linux variants you mentioned, but extremely stable.
so what 'flavour' of linux would be closely identifiable to OSX's own terminal and runnings?