The question is testing the understanding of English grammar terms related to conditional sentences. The correct answer is 'C. Unreal condition clause' because it specifically refers to sentences that describe situations that are not real or are hypothetical. Option A is incorrect because it refers to real conditions, which are not imaginary. Option B is incorrect because it combines elements of both real and unreal conditions, which is not the focus of the question. Option D is too general and does not specifically address the concept of imagination or unlikelihood in conditional sentences.