What's the difference between "tio" and "tiu"?
"Tio" refers to an unspecific thing or a general class of objects; "tiu" refers to a specific thing, person, or phenomenon. You'd say "tio estas kato" and "tiu estas Fluffy".
It accepted my answer, but now I'm thinking that the sentence suggests "tiu." If someone is pointing at an object when asking the question, whether they know what the object is or not, isn't "tiu" more logical than "tio"?
Why is "estas" required here? Seems to me that "Kies tio" would be sufficient for a phrase like this.
Because you need a verb.
"Whose that" doesn't make sense. You might be thinking that "whose" is a contraction of the words "who" and "is", but it's not. The correct contraction is "who's".
I'm not mixing up contractions. I thought that maybe "estas" could be omitted because I find it hard to come up with other words that could fit the sentence.
In English, you wouldn't say "Whose that?" you'd have to say "Whose is that?"