Translation:There are six large bridges over this river.
No clue, guys! "That's just how it works" is not an answer you would want to hear from an English speaker if a person was trying to learn English! This information belong in tips, not threads for individual examples!
And I don't care if there is a thumbdown on this - I have gone through top to bottom, left to right, and Choices 2 is now the worst section thus far - with Choices 1 now second!
I like to think about postpositions as suffixes which started a life on their own. Just like the 'z' in ez and az assimmilates when a suffix that's starting with a consonant is added (abban, ennél, and so on), it assimmilates to nothingness when there's a postposition with a consonant following.
Well, the reason is simple: whenever you have a demonstrative pronoun (ez/az - this/that), its grammatical baggage copies that of the noun it represents. In this case:
Ez a folyó - this river
Ebben a folyóban - in this river (same suffix, but different vowel harmony)
Ezekhez a folyókhoz - to these rivers
Ez előtt a folyó előtt - in front of this river
E fölött a folyó fölött - above this river. If the postposition starts with a consonant, ez is reduced to e and az to a.
The tips and notes are only available in the web version of Duolingo -- which you could in theory access from a phone if you used the browser to access the website rather than the app.
It's a pity that the tips and notes aren't available in the app, because this way, app users are missing out on a lot of valuable background information that the course maintainers have provided. (And most probably don't even know that they're missing stuff! Especially iOS app users who don't even have access to sentence discussions.)
As for the double fölött, the rule seem to be that if you have ez, az, ezek, azok (this, that, these, those) + a(z) NOUN, then if the noun takes a postposition or a case ending, so will the the ez, az, ezek, azok.
For example, ez a folyó "this river" + fölött = e fölött a folyó fölött "over this river".
ez and az drop their -z before a consonant -- it would stay on if the postposition started with a vowel, as in az alatt a fa alatt "under that tree", from az a fa "that tree" + alatt.
An example with cases: azokban a házakban "inside those houses", from azok a házak "those houses" + -ban.
With case endings, the -z or ez and az gets assimilated, so "inside that house" would be abban a házban, where the -z turned into a -b before the -b- of -ben.
From what I've heard, the current version of the website is sensitive to screen size, and on small screens it will look like the app (including not displaying the tips and notes) -- unfortunately, in my opinion.
If you get the chance, visit the website with a computer or at least a tablet.
Miran, fölött is a postposition, which means it only appears together with a noun, as in "valami fölött" - "above something". This construction is used to talk about the space above that specific object.
Fent is an adverb and does not associate with a noun. It refers to a general "above" (or "up" or "upstairs"), without referencing a specific object. English has a bit of trouble conceptualising those adverbs.