<p style="">The correct answer is (C) ours.</p><p style="">This sentence requires a possessive pronoun that refers back to something previously mentioned, which in this context is the "budget planning."</p><p style="">(A) "us" is an objective pronoun referring to the object of a verb or preposition, and it doesn't indicate possession.</p><p style="">(B) "our" is a possessive adjective that needs to be followed by a noun, which is not present here since the noun "budget planning" is implied, not directly repeated.</p><p style="">(C) "ours" is a possessive pronoun that stands alone and shows ownership, fitting the requirement of the sentence. It replaces "our budget planning," preventing the need to repeat words and maintaining coherence in the sentence.</p><p style="">(D) "ourselves" is a reflexive pronoun used to refer back to the subject of the sentence, which is not the case here.</p><p style="">So, the sentence should read: "The marketing department has finished its budget planning for next year, but we in the Research and Development department haven't completed ours yet." This indicates that the speaker and their department have not yet completed their version of the task (budget planning) that the marketing department has finished.</p>