"The boys want banana" doesn't really make sense. "The boys want a banana" should be accepted.
Although Duolingo's option "The boys want bananas" is the best translation, the English sentence "The boys want banana" can make sense; it could be the answer to the question "What flavour ice cream do the boys want?" when the boys want banana flavour ice cream.
We are used to say that in singular, meaning bananas. The quantity is not defined.
it should be the boys want banana If you say bananas it should have an s at the end of the word banana
Being fluent in both languages I can say for sure the best translation for "eles querem banana" is "they want bananas".
When the singular has no article, such as in this example, it's often used in a general sense, just as the phrase "eu gosto de maçã" means "I like apples", not "I like an apple". In English this general sense is achieved with the plural, but in Portuguese, as you can see, it's with the singular.
To say "they want a banana" the best translation would be "eles querem uma banana". Remember translation is not about being literal, but getting the closest meaning.
Please correct me if I'm wrong, but I'm starting to think the preferable way to translate between languages is for common usage (over literal translation). I read in another discussion in Portuguese the general use of a word is usually singular (the boys want banana), while in English we tend to use the plural for general use (the boys want bananas). "The boys want a banana" would be "O meninos querem uma banana", no?
It seems clear that the question needs to be amended. The speaker says banana, but the listener cannot comprehend that a group of boys would consider sharing a single banana, so he/she writes bananas.