Adjectives are invariable. The moment you make it plural, it's no longer an adjective. It becomes a pronoun, and the sentence will be understood as: These are books.
Ektoraskan seems to state that bu is an adjective, and that bunlar is a pronoun. Surely bu is a pronoun.
"this book," in this case, it is an adjective.
"This is a book," here is it not.
If "this/these" comes direct before a noun or an adjective modifying a noun, it is also an adjective. If it is by itself or is followed by "one," it is acting as a pronoun.
Ektoraskan says that when bu becomes plural, it is no longer an adjective, it is now a pronoun - thus bunlar kitaplar means these are books. AlexinNotTurkey says if 'these' comes direct before a noun, it is an adjective. In bunlar kitaplar, bunlar is direct before a noun, and is therefore an adjective. It has previously argued that 'bu kitaplar' means these books, while bunlar kitaplar means these are books. There is a contradiction here!
- Bu kitap = This book (Bu = adjective)
- Bu, kitap. = This is a book. (Bu = pronoun)
- Bu kitaplar = These books (Bu = adjective)
- Bunlar kitap = These (ones) are books (Bunlar = pronoun)
As a native Turkish speaker I just wanted to check the questions and this translation is not right. Because since it starts with a capital letter it supposes to be a sentence thus it can not be an adjective, it must be a pronoun. "these books" means nothing.
"These books"="bu kitaplar" is the beginning of a sentence as: "these books are annoying". Jut take it as an exercise.
Well, how does somebody suppose to understand it? They could simply add "..." or complete it in order to show that this is not a sentence, but beginning of a sentence. Learning without a teacher is not easy by itself. No need to make it harder by using misleading sentences. I don't think we can say "just an exercise", because Duoling uses only exercise. So teaching something wrong in the exercises can not be something acceptable in this system.
Please, look the consonant mutation rules on duolingo turkish gramar portal or there: https://www.turkishexplained.com/conmut.htm and there: https://www.turkishexplained.com/Turkish-Consonant-Mutation.htm