"Who is that Grandmother?"
Translation:ʻO wai kēlā tūtū wahine?
What's the difference between using 'o ia and kēlā as "that"
They have very similar meanings, in which the nuances would best be described by a grammar lesson beyond this course. I suggest picking up a grammar book or looking at this one, online for free: http://www.ulukau.org/elib/cgi-bin/library?e=d-0hawaiiangrammar-000Sec--11en-50-20-frameset-book--1-010escapewin&a=d&p2=book
That is also correct. Tūtū wahine, kupuna wahine, and kupunahine are all accurate translations of grandmother.
"ʻO" is used before the subject if it is a pronoun, proper name, or the interrogative "who." Then, the rest of the sentence's details can be added on: Examples include: ʻO wai kēlā tūtū? == Who is that grandparent? ʻO ia kēlā tūtū. == She/He/It is that (aforementioned) grandparent. ʻO Keola kēlā tūtū. == Keola is that grandparent / That grandparent ('s name) is Keola.