I think these two sentences are correct
"she had gone to buy fruits."
"the mother feeds the poors"
but it was given that the plural used here is wrong here;in fact, it was written that these words always exist in singular form.
'Fruits' is normally used when we are discussing the different types-- 'among the fruits, citrus fruits are higher in vitamin C than other fruits'. In daily conversation, 'fruit' is usually uncountable-- 'she had gone to buy fruit'. There is some individual variation in this usage, however.

'Poor' is a different case: it is the nominal use of an adjective, and these are uncountable. 'The mother feeds the poor', 'the rich get richer', 'the sick and wounded were left to die'.
thank you ,sir!