Well, the question is incorrect, anyway - RG is also available in the Evaluation Edition. So that "only" without this version mentioned is IMO misleading.
Maybe, but the answer really is obvious (how can anyone get it wrong even by making an intelligent guess? After all, SQL2k8 is carefully packaged so that all major new features are in developer edition to get the developers hooked and in enterprise edition to ensure that those hooked developers genrate the maximum revenue for MS).
What I really don't like about this question is the appalling carelessness: a question with no verb, and an explanation that says that SQL 2k8 is supplied only in developer and enterprise editions are serious defects compared to the trivial mistake of thinking that evaluation edition doesn't exist (evaluation is of course, like developer edition, intended to get us hooked on things that will make us buy enterprise edition; or am I just being cynical here?).