I think that correct answer is some fruit juices are very acidic.

Native speakers tend to use plural forms (even with typically uncountable nouns) when distinguishing between species, varieties or forms of things. Let us consider your example. "There are different kinds of juice. Some of _____ is/are quite acidic." What should you put in the space? would you use is or are? how does the pronoun inform this decision? We could say "There are different kinds of juice. Some of them are quite acidic." 'them refers back to juice as a Plurality - so, juices. This lends weight to the argument that both juice/juices can be correct, e.g.: There is some juice on the table. // There are some juices on the table. In the first sentence, juice refers to the liquid beverage; thus, it is uncountable. In the second sentence, juice refers to the different varieties of juice (e.g., apple, grape, pineapple, etc.), and therefore, is considered a countable noun. Now to your concern - we want an utterance that feels authentic to the Native ear. Both of your options are suitable and no-one would notice in general conversation. If you are referring to 'juice' in a general way, we would use the singular / uncountable. When referring to varieties, we lean toward to plural.

在这个短语中，"fruit juice" 是不可数名词，因为它指的是一种液体，而不是具体的果汁种类或数量。 “Some” 在这里表示 “一些"，并不表示具体的数量。因此，"some fruit juice” 表示 "一些果汁"，而不是 “一些杯子的果汁” 或 "一些种类的果汁"。如果要表示具体的数量或种类，可以使用可数名词的形式，例如 “two glasses of apple juice” 或 "three types of fruit juice"。

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Benjamin Pike gave the perfect answer. That's the answer!