we say cubanas because it a female and it's more than one female if it was one female we would say ella es cubana since it's more than one we say cubanas.
The reason it’s c is because when you say ellas we are talking about a group of females, so the sentence “ellas son” is they are. What you need to find is a word that is feminine and plural, which is cubanas.
What are you trying to describe? But this translated into english is Decide the location of each adjective according to the meaning. There is a fort in Old San Juan.
