Since ‘fruit’ is also plural (see: https://www.collinsdictionary.com/us/dictionary/english/fruit) Many fruit is grammatically correct, although ‘a lot of fruit’ does sound better.
Yes, Daleswords: the singular is "fruit" (used as a mass noun) and the plural (countable) is "fruits". But grammatically, the determiner "many" can only be used with the plural form of a noun (e.g. "fruits"), whereas "a lot of" can also be used with singular uncountable nouns (mass nouns). That's why "a lot of fruit" sounds better. See https://en.wiktionary.org/wiki/many#Usage_notes
Actually, there is no difference in meaning. "A lot of" is informal and "many" is formal. For example:
"A lot of people think that ..."
"Many people believe that ..."
However, you can use "a lot of" with singular or plural nouns, whereas "many" only works with plural nouns. So when pahash suggests the translation "a lot of fruit", it is because you can't use "many" with a mass noun (singular, uncountable), e.g. "fruit".
But it could be correct to use the plural "fruits" in this sentence, if it refers to many kinds of fruit. For example:
"Many fruits, such as bananas, figs and cherries, have a high sugar content."
Nouns for classes of foodstuff (fruit, meat, cheese, etc.) are usually uncountable, but they take a countable sense when we talk about different varieties, e.g. "a wide selection of cold meats and cheeses".
The lack of context makes me follow my gut reaction, as pahash did. It might be that the Swahili course contributors translated the Swahili plural "matunda" literally as "fruits", whereas English would most often use the singular "fruit" (e.g. "There is a lot of fruit in the bowl") to refer to a large amount rather than many kinds of fruit.
Point taken, martinemika. But actually, I see this as a Swahili translation issue, because it seems that Swahili doesn't distinguish between "a lot of" and "many", as English does. You can say:
"Ninakunywa maji mengi" ("I drink a lot of - a large volume of - water")
"Ninakula matunda mengi", which could mean "I eat a lot of fruit" (e.g. kilos of it) or "many fruits" = many kinds of fruit (e.g. oranges, mangoes, bananas).
It looks like this is one of those questions where the correct translation is only resolved by the context, and Duolingo questions often lack a context.
NB: An interesting complication is that Swahili uses the plural for many nouns that would be singular and uncountable in English, e.g. maji = water, maziwa = milk, matope = mud.