At last we’ve found our book, but where is ___? ?

In the sentence 'At last we’ve found our book, but where is ___?', the context indicates that '___' should be filled with a word that refers back to the person being addressed and relates possession.

The options presented include:
- A) her
- B) your
- C) you
- D) yours

To determine the correct answer, we need to analyze each option in terms of grammatical correctness and meaning.

- Her (Option A) is a pronoun that indicates possession but would imply a third person. It does not fit the direct address in this context.
- Your (Option B) is a possessive adjective that modifies a noun, but it needs a noun to complete the thought. Without a noun following 'your,' this option is incomplete.
- You (Option C) is simply the pronoun for the person being addressed and lacks specificity regarding possession.
- Yours (Option D) is a possessive pronoun that directly indicates something belongs to the person addressed, fitting seamlessly into the structure of the sentence. It correctly completes the thought, implying that the listener has a book that matches the one referred to.

Thus, 'yours' correctly conveys possession and grammatically completes the sentence, making it the only suitable choice.