Good questions! I was working out some of these semi-familiar forms as well. The notes have the singular forms as איזה (MS) and איזו (FS), and the plural form as אילו.
Duoling's Hebrew "Tips and notes" state:
"You will find that in Modern Hebrew people very often just use the singular masculine form, איזה, instead of the plural אילו, and to a lesser extent, איזה instead of the feminine איזו for feminine nouns. You should try to refrain from this habit, although we do often allow these technically incorrect answers due to the frequency of occurrence among native speakers."
Edit: I corrected the above forms since I had misread the notes.
You say "You should try to refrain from this habit" - why? In Modern Hebrew, איזה is the default form for the question "which", so if you, as a native or fluent speaker, know your thought begins with which you say איזה before you even think about grammatical gender and number of whatever follows. Of course, practicing grammatical agreement will help new learners learn how agreement works in Hebrew, but refraining from speaking in a manner used by most native speakers sounds like less than useful prescriptivism.