I think the proper way, in a day-to-day usage, would be "estão". For an exemple, if i ordered an apple in a restaurant, and, after tasting it, comentend with someone about it being good or bad i would say "As maçãs estão boas".
Well. Two points you can't use "bom": 1- you have a definite, feminine, plural article: AS. 2- You have a feminine, plural word: MAÇÃS. So you just have "as maçãs são boas". "Bom" is for masculine, singular words or general context, like in "maçã é bom".
Wow. So you're saying that even though "maçã" is feminine, if you're making a generalization like "apples are good" as opposed to a specific assessment "this apple is good", you'd use the masculine form of "bom"? Am I understanding correctly?