which is the appropriate indefinite article for the word libretas?
It is unas libretas. Unas is femenine and plural indefinite article, and it match with libretas. You do not have more options @Crissy15 in this case. @Crissy15 if you say las libretas you are using the femenine and plural definite article las. if you say unas libretas you are using the femenine and plural indefinite article unas. Crissy15.