What happened at Mount Doom?

[The Saucepan Man]

And with that, even as his eyes were lifted up to gloat on his prize, he stepped too far, toppled, wavered for a moment on the brink, and then with a shriek he fell.

On the basis of the "facts" as presented to us, the only thing that we can say for certain is that Gollum slipped. There is certainly material to suggest that Eru gave him a little nudge, but the reader is by no means bound to accept that. Indeed, having no knowledge of Eru when I first read the tale, it would have been impossible for me to draw such a conclusion. Option B is therefore dependent upon the individual reader.

Option C is most certainly out of the question as it runs contrary to the stated "facts" and, as for option D, well I can imagine that an alternative scenario, which retained the essence of the story, would have been possible.

Option A it is then.