It looks like you have already done a good job by eliminating responses A and B. To me, it is obvious that answer D refers to everything that the author is talking about in the passage. Answer C can be eliminated since the passage also discusses profit motive in education, something that may or may not involve online education. Choice E is a little more difficult to eliminate. But if I was trying to assign choice E to a passage, I would expect to see that passage address several pros and cons of online education. This passage operates more like a chronological account of the advent and development of online education, which is why I would say that choice D is the BEST response. I hope this helps.