Why can't C be correct?

Hi @Joyce, Thanks for the question! So let’s take a look at what the stimulus is telling us. First, we’re told that speakers of languages with more words for family members have a more finely discriminated kinship system than English speakers do. We also know that the number of basic words for colors (red, blue, yellow) varies from language to language. So, the argument concludes, speakers of languages with fewer basic words for colors than English can actually see fewer colors—they are perceptually unable to distinguish as many colors as speakers of English. Now we’re asked to find a necessary assumption of the argument. Take a look at (C), which tells us that every language makes some category distinctions that no other language makes. Is this a necessary assumption, does the argument have to assume this? No, and the extreme wording in “every” language and “no other” language should alert us to the fact that this is unlikely to be the answer choice. Let’s try out the negation test and say that not every language makes some category distinctions that no other language makes. Does this weaken the argument? No, not at all, and so we know it’s not a necessary assumption of the argument. So (C) is incorrect. Hope this helps! Feel free to ask any other questions that you might have.

Isn't this a sufficient premise question? Not a necessary premise question?