Hahahaha! All I can say is "I'll get you for this!" LOL. Okay, people. I have delved to the deepest depths on the issue of collective nouns in English, countable vs. uncountable, fruit in general, multiple kinds of fruit (but not all fruit), and the differences between UK and US English (and yes, there are differences there, too). The result is that there are very many ways to think about what we read in the French and what seems to be very straightforward (fruit has seeds) until we try to put it in idiomatic and correct English. Background: the Englisih "fruit" as in "fruit in general" is a collective noun and takes a singular verb (UK and US sometimes handle this differently). English "fruits" (e.g., the apples, oranges and pears) is a plural noun and takes the plural verb. The plural "fruits" is countable in French but the plural "fruit" is uncountable in English. When you want to refer to multiple kinds or multiple pieces of fruit that have been previously mentioned (but not fruit in general), one would say "the fruit" (with the definite article) to indicate that there is more than one (but not all) as opposed to the French in which "les fruits" could be fruit in general or multiple pieces of fruit of one or several kinds. In English, the plural "fruit" (sans article) will be assumed to be fruit in general, or (multiple pieces of) one kind of fruit, as in "some fruit". I'll skip the partitive here since it's not relevant. There are also differences in how UK English handles collective nouns (may be treated as either singular or plural) whereas US English treats collective nouns as singular (only). After this, if anyone still wants to insist that "such-and-such way is not correct", rest assured that I have turned this every which way. While some of the outcomes may not "sound" quite to my liking, and indeed, I would not personally say them, yet there are more more ways to render this than might first appear.
Thanks for the detailed reply. I am not one to suggest excluding correct but odd-sounding answers! I don't want to be pedantic, and I don't want Duo to be pedantic (certainly not about English), but this does seem like a case in which it would be reasonable to add to the accepted answers those which reflect the language's quirks and our understandings of the relationships between fruit, fruits, and their seeds.
P.S. I actually have a Duo-relevant exception to the "US English treats collective nouns as singular (only)" rule: the police. I've never heard "The police is..."; it's always "The police are...". And yet the singular is (or at leastl recently was) Duo's only accepted answer.
"Fruits" is most certainly an English word. I can say "Apples and oranges are fruits". The correct translation would be either "Fruits bear seeds", or "Fruit bears seeds". The problem with Duolingo is that their English translations obviously come from non-native English speakers, making them awkward and confusing. They need to have a native speaker edit their translations.
But you can refer to several pieces of fruit as "all the fruit". All the fruit on this table have (plural) seeds. But not, The fruit have seeds.
This may also be another of these Trans-Atlantic differences. In Britain, they use plural verbs for collective nouns like army or council - The army are maneuvering. The council are deliberating. So possibly they also say The fruit have seeds.