'Meat' should also be accepted as it is both singular and plural.
It should be, but carneS is plural and carneM is singular. I think the app wants us to differentiate these
I suppose the sentence means that "The merchant sells various kind of meat". Otherwise caro would have been used in singular.
Yes, but it is given that a merchant who sells meat sells various kinds of meat.
I don't think it's a given unless the sentence gives context for it (eg. fish and beef).
It is a given because of the meaning of meat and caro. The plural means different kinds.
Of course, that's the obvious truth. I was just stating the difference in meaning between carnes (which is used in this sentence) and carnem, to justify the given translation.
So if carnem is singular, and carnes is plural...
What is caro?
Specifically, "carnem" is the singular form for the accusative case, and "carnes" is plural for both nominative and accusative. The form "caro" is nominative singular. (Full conjugation here.)
Not in some context, if it's a horse butchery. And selling meat (in general), only one kind of meat, and several kind of meats have a different meaning.
What's wrong with "The vendor sells meat." The word "meat" is one of those collective nouns (like sheep and fish) in English, isn't it?
"Meats" emphasizes "different kinds of meat".
Merchant ≠ salesman?
No. A merchant usually has possession of the goods sold. A salesman is usually a middleman, who usually sells other people's goods/services.