From my musician friend Līhau who plays upright bass, I learned two other words for this instrument as well:
What is the difference between “kaula” and “ʻaha”?
They are both words used for "string". In these Duolingo exercises, you can use them interchangeably to refer to the strings of an instrument.
Wiola ku nui and wiolina nui , and wiolina ku nui. "elima 'aha o ka pila wiolina nui
If you are going to leave messages for the group, please translate them so we don't have to look up what you mean by your comment?