Translation:Which of these strawberries are red?
Yes, that's correct. It means "Which one of these strawberries is red?". The use of plural "lesquelles" is prompted not by "fraises" (which has to be plural or there would be no choice and the sentence would make no sense), but by the meaning "Which ones are red?". The singular "laquelle" means that one strawberry is red and we want to know which it is, the plural "lesquelles" means that more than one strawberry is red, and we want to know about all of them.
Must the French be written "Lesquelles de ces..."? Could it not also be written "Quelles de ces fraises sont rouges"? I'm trying to understand why one would need to say LESQUELLES and not simply QUELLES (since "WHICH of the strawberries are red" would (in English) mean the same thing as "WHICH ONES of the strawberries are red"). Thank you!
The grammatical explanation is that quel is an interrogative adjective, and "lequel* is an interrogative pronoun, so they are not interchangeable.
More straightforwardly, we say "Quelle surprise!" ("What a surprise!") where quelle means whator "which" or "Lequel des chiens? where lequel means what/which one* (of the dogs), or, more longwindedly, "What/which dog of the dogs?".
In other words, quel = what/which and lequel = what or which one. In English you can often leave out the one bit, but not in French.
- lequel is the masculine singular form. It's used when referring to a single object known to be of masculine gender, eg. "Lequel de ces documents est rouge ?" ("Which one of these documents is red?", that is to say that one of the documents is red and I want to know which one it is).
- lesquels is the masculine plural form. It's used for a plural object of masculine gender, eg. "Lesquels de ces documents sont rouges ?" ("Which ones of these documents are red?", that is to say that multiple of the documents are red and I want to know about all the red ones). Note that the verb and adjective change to plural to match "lesquels".
- laquelle is feminine singular. It's used the same way as "lequel", except with feminine gender, eg. "Laquelle de ces fraises est rouge ?" ("Which one of these strawberries is red?", one of the strawberries is red and I want to know which one).
- lesquelles is feminine plural, used the same way as "lesquels" but with feminine gender, eg. "Lesquelles de ces fraises sont rouges ?" (Which ones of these strawberries are red?", multiple strawberries are red and I want to know which ones). Again, note the change to plural in the verb and adjective.