I've never really thought about it too hard, but I don't think it's like a lot of these concept drawings of orbs or colums or organicy looking things floating in some nether realm - to me its a machine or a ship .I don't like all these modern notions taken from the novels that TARDIS' are organic. If you watch the classic series it's clearly a machine. Oh sure, it posesses a sentience of some sort, but it's not organic imo. The interior simply occupies a different dimensional space.
I think it's option 5.