"Which will you use?"
Translation:Kiun vi uzos?
Only nouns can be objects. I believe only the -o, -u and -n correlatives are nouns, and the rest are adverbs, adjectives, or particles. For more on correlatives, see: http://donh.best.vwh.net/Esperanto/correlatives.html
Interesting example, but isn't that effectively a noun? Not only is "one" or "item" implied, but it's usually said in the context of a concrete set of items -- there's a bunch of dresses, or toys, or confections, and one of them is blue. Same with, for instance, "la Angla" -- the English person. There's a specific group of people, and one of them is English.
Yes, the demarcation between nouns and adjectives is a bit fuzzy. The umbrella term is Nominal. You can also use nouns like adjectives to modify other nouns.
In languages in which nouns are declined, adjectives are typically also declined, and in much the same way. This includes Esperanto, where all nominals (nouns and adjectives) must be marked for accusative, where applicable.
That would be "what" rather than "which".
"Which" implies a specific noun (e.g. "which knife, which book") which you have in mind even if you don't state it explicitly; it's kind of like an adjective in that respect that is associated with a noun. That maps to "kiu".
"What", on the other hand, is more like a pronoun; it doesn't necessarily have any particular noun in mind that it modifies. (Though English can also say things like "what book", which confuses things.) This more non-specific, pronoun-y use corresponds to "kio".
Sort of -- kiu can be "what ....." or "which .....", and it can also be used by itself, in which case it can mean "which person = who" or "which one". (Kind of like la blua would be "the blue one" in English -- English generally needs a dummy noun "one" after this kind of word.)