Hmm, interesting. I don't disagree, but I'd say that all these terms are being used in relation to the person speaking AND apply to what the person in that position calls someone. Let's say you call your grandmother Ney-ney and your grandfather Pappi. You'd say "Pappi is Neyney's husband." I admit the situation here is not exactly the same, because Bua and Phupha are terms for anyone's aunt/uncle. And yet, you've got at least three different terms for aunts and uncles in Hindi so you can't just say "an aunt" -- you have to be specific.
You could say (to fit your criteria) "A mother's sister's husband is an uncle." It seems thought that if the native Hindi speaker is translating this, they would personalize it. Just a quirk of the language that doesn't translate perfectly.
Still, I agree with you that some accommodation needs to be made in the answer/reply options.
I'm not sure what you are getting at. Yes, the Hindi terms have information encoded in them that the English terms do not. If that information is not vital, however, the English terms that cover that relationship (along with others) is the term that should be used in translation. If I were translating /buaa/ and /phuphaa/ into English, though, I would use the terms "aunt" and uncle." This sentence, therefore, could be either "the aunt's husband is the uncle" or "an aunt's husband is an uncle." Both those sentences make grammatical sense in English and could be true, so that is how I would translate this sentence. There may be a dialect of English in which "aunt" and "uncle" are used as terms of address, like "grandma" and "grandpa," as they apparently are in Hindi, but it sounds very odd in the dialects in which I am conversant. We might say "Auntie," but I have never heard "Uncle" without a name attached, e.g. Uncle Bob or Uncle Bill. Consequently, I am simply pointing out that, if you were interested in learning English for use in the United States and the United Kingdom, the sentence "Aunt's husband is uncle" does not make any sense. Indeed, it is grating even to write.
Again, I agree with your recommendation.
Ah, but you do know a dialect of English where Aunt(ie) and Uncle are used as an address, along with mom, dad, grandpa, sister, etc: Indian English! The course is (perhaps inadvertently) teaching Indian English, which is at least as useful in India as Hindi :-)
If they were to add /jī/ after all these terms in the lessons it would be more realistic.
In fact, I do not know Indian English, though I am very happy to learn some of it. If those are the forms of address used, then the correct answer should stay, it seems to me, though "the" and "an" should be accepted.
It is kind of odd to me that they have not introduced "ji," since it seems to be quite common in instruction books and in the spoken Hindi one sees in films.
I don't care about precise English grammar per se. I just know that I need to know the meaning of what I am saying. If the English is puzzling, or very unlikely, or if it is impossible to imagine saying the English sentence, my brain rejects the Hindi as meaningless. I might memorize it, but not be able to acquire it as usable language.
Would you suggest an alternate English translation?
Imagine the sentence is "Mom's husband is Dad." How else would one say that? In the example sentence, it sounds a bit funny because American/British etc. English speakers don't give "Aunt" and "Uncle" the same status as "Mom" and "Dad." But Indian speakers do... I'm not sure how this could be said differently in English aside from adding "my" or "a," which, because they are not present in the Hindi sentence, would be confusing for the learner. (In that case, we'd be having learners asking "Where does it say 'my'?")
Wouldn't the होते हैं be accurately reflected by using 'an': An aunt's husband is an uncle. It would be sort of defining 'uncle' for someone who is learning English. Without the article, a mother might be explaining their own family to a very young child: Aunt's (your aunt's) husband is uncle (your uncle). But it sounds stiff and unnatural now , something from a nineteenth century novel; not at all the way a modern mother would speak to a child. I am not against sometimes using unnatural English to teach something specific about Hindi. But I can't say that my poor brain can discern what this odd English locution is teaching, since it doesn't correspond to any current usage.