Translation:My mom likes eating watermelons the most.
Imagine the following situation: a vendor gives you several options of fruits for you to buy to your mom. Provided that you only have a few bucks, you have to think about what your mom prefers. The "correct answer" (there are more than one in reality) fits here, because among the given options, your mom likes watermelon the most. But the sentence you provided doesn't fit here because it doesn't apply to a limited set of options but for all possible options in the world.
So is it the mother that likes watermelons more than anyone else, or is it that out of all the things that she likes to eat, watermelons come top? The answer has specifically come up with the comment that the definite article is needed. In the English that I speak as a native, it is unnecessary for either of the meanings.
西 (xī) = west
瓜(guā) = melon/squash/gourd
西瓜 (xīguā) = watermelon
Squash and gourd are kinds (families) of fruit among which:
黄(yellow)瓜 = cucumber
青(green)瓜 = cucumber
南(south)瓜 = pumpkin
木(tree)瓜 = papaya
苦(bitter)瓜 = bitter melon
The native range of the watermelon extends from north and west africa to india, so it was introduced to china from the west. The pumpkin is introduced to china from the south.
Your answers are marked by a computer which compares your answer to a list of correct answers someone has to enter into the computer. It is simply not possible to anticipate all possible non-grammatical English language answers and add them to the list of "correct" answers. The list would be almost endless. You need to try to give grammatically correct answers as these are the only ones Duo will normally have entered. Duo is not criticising your English or trying to teach you English, they are just trying to match your answer with a list of correct answers to see if you have understood the Mandarin sentence.
In English, yes, there is a difference. 'Prefer' suggests that two things have been compared. 'I like this one better than this one - I prefer it'. Whereas 'like ... the most' puts one above many others. I don't know if Chinese has the same issue, but in English grammar (and it may not be the same in US English), when two things are compared a different adjective is used from when three or more items are compared. Hence 'He is the taller of the two brothers' whereas 'He is the tallest of the three/four/five etc brothers' I think that the difference you have highlighted is part of the distinction that is made in English between dealing with two items and several.
I beg to differ. I could reasonably say, "I am taller than both of you;" therefore there are three objects in comparison here. I could also say, "I am shorter than a giraffe, an elephant, and a polar bear." I see absolutely no reason why one could not say, "I prefer watermelon to apples, oranges, and pears," thus having more than two items in comparison. I would agree that the sentence Duo gave should probably not be translated as, "My mom prefers eating watermelon," but that is because "prefers" does imply comparison, and we have no contextual evidence of there being a specified other thing/group of things being compared. When you use "likes the most," you do not need another thing to be there for comparison because it is already implied that you are comparing to everything else. "Prefers" is more similar to saying "likes more" rather than "likes most," so we would need evidence of a comparison being made to use "prefers."
I think it would be better as "my mom likes eating watermelon best", but that want accepted when I tried it... Or "my mom most likes watermelon", which is a bit awkward, but at least doesn't suffer the problem of possibly also meaning that out of a group of people, my mom is the biggest fan of watermelon.....