Odgovor na pitanje je u knjizi.

to be

Breakdown of Odgovor na pitanje je u knjizi.

• u: in
• knjiga: book
• pitanje: question

Questions & Answers about Odgovor na pitanje je u knjizi.

What does each word correspond to in English?

• odgovor = answer (noun, masculine)
• na = to (here: marks the target of the answer)
• pitanje = question (noun, neuter)
• je = is (3rd sg. of biti “to be”)
• u = in
• knjizi = in the book (locative singular of knjiga, “book”)

Which cases are used in the sentence, and why?

• Odgovor: nominative (the subject).
• na pitanje: accusative (the preposition na takes the accusative in this “answer to” pattern).
• u knjizi: locative (static location with u “in” uses the locative).

Why is it na pitanje (to the question) instead of something like o pitanju (about the question)?

Croatian uses the set pattern odgovor na + accusative to mean “answer to.”

• odgovor na pitanje = answer to the question
  Use o + locative to mean “about”:
• rasprava o pitanju = discussion about the question/issue

Could I say odgovor pitanja to mean “the answer of the question”?

No. odgovor pitanja sounds wrong. Use .Genitive after is used for possession/agent, e.g. “the professor’s answer.”