Explanation

Marble is metamorphic. Granite is igneous. Sandstone is sedimentary. The question asks for 'not a metamorphic rock'. Both B and C are not metamorphic. Option D says 'both A & B', which is incorrect because A is metamorphic. Therefore, the current answer D is wrong. Between B and C, both are valid answers to 'not metamorphic'. However, usually MCQs have one best answer. Let's re-read carefully. 'Which of the following is not a metamorphic rock?'. Marble (Metamorphic). Granite (Igneous). Sandstone (Sedimentary). Options: A=Marble, B=Granite, C=Sandstone, D=both A & B. If the question implies only one answer, there might be an error in the options or the question. However, looking at standard geology MCQs, often Granite is contrasted with Marble as the classic igneous vs metamorphic pair. Sandstone is also not metamorphic. Option D claims A and B are not metamorphic, which is false since A is metamorphic. So D is definitely wrong. The current answer is D. I need to suggest a correction. Since both B and C are correct answers to the question text, but usually in these flawed MCQs, 'Granite' is the intended distractor against Marble. Or perhaps the question meant 'Which is a metamorphic rock?' In that case A would be correct. But taking the text as is: Not metamorphic = Granite or Sandstone. Option D is logically false. I will suggest B as it is a common counterpart to Marble in such quizzes, though C is equally valid scientifically. However, sometimes 'Sand stone' is considered less distinct in simple quizzes than Granite vs Marble. Actually, looking at the pattern, if the answer key says D, the question might be flawed or the options are messed up. But I must verify correctness. Current answer D is incorrect. Suggested answer: B is a strong candidate for the intended answer in many contexts, though C is also correct. I will pick B as the suggested correction, noting the ambiguity if needed, but the prompt asks for strict JSON. I'll set confidence high that D is wrong and B is likely the intended answer due to common pairing with Marble.