The forest and region, not just the river.
But only a very little context is enough to peg it as any of the others.
Why uncapitalized? Wouldn’t an Amazon be capitalized like a Pict or a Goth?
Other. All of the above.
With no context I would wonder which of the three things you listed was being talked about.
“amazon” - any Amazon-like woman.
“Amazon” - the online bookstore.
I think rainforest.
Absent any other context: river/rainforest first, then online merchant, then female warrior.
Same here.
This, except I would add the definition of a large, strong woman, not limited to the warriors, to the last definition.
I first thought of the bookseller, then of the female warrior. It wasn’t until I saw the poll that I was reminded there was a river by that name. Voted ‘other’, but I’d probably say the bookseller if I had to choose one.
Amazon originally came from Greek myths and is a word meaning “without breast”. The legend was that women warriors cut off one breast to make drawing a bow more effective. So lower case is just fine. The only documented case of effective and feared female warriors are those of the Fon tribe of Benin.