A: If you use "These" then it must be as: These are bags (carts, boxes) of apple. B: And if you use "This" then it must be as: This is a bag (cart, box) of apples. So if the bag is plural then apple must be singular, and if the bag is singular then the apple must be plural as in the cases A, B

"This" and "is" are correct. "This is a bag of apples." You have the singular term "a bag". The singular "is" goes with singular, countable nouns, as does "this". In this sentence, "of apples" modifies "a bag" and it is not necessary. The "bag" is the toic f the sentence. You could just say "This is a bag.". If you want to use " these" and "are" you need to change the format of the sentence and talk about the apples, not the bag. "There are apples in that bag.".