Sort of funny, right? hoaloha is an O-Class noun, so I guess the shorter, less-intimate version hoa is also an O-Class.
the translation of "hoa" is friend... the correct pronoun usage of the possessive pronouns "koʻu" versus "kaʻu" is determined by agency or control of a circumstance or object... Because I control "who" I choose for my friend, "kaʻu" hoa is the correct translation... Koʻu hoa hānai would be correct because I have no control over who is my cousin...
In this lesson, DLHawaiian often does not offer choices (in the choose 1 of 3 queations) that let us choose between kino ʻō or kino ʻā options. Usually one (the right one) is given so we can't learn from our mistake. More creativity in developing questions could help this.
As a test creator, I do tend to prefer for multiple choice a variety of 4 choices where there are 2 reasonable possibilities. It offers some chance to ponder what one has learned. I think Duolingo has its reasons for doing 1 right answer only with the rest being obviously wrong in order to make the lessons progress faster... maybe. not sure.