Explanation

Organic soils are primarily classified as peat (undecomposed) or muck (decomposed). Since both A and C fit the description but option D only includes 'a & b' (Arid soil is inorganic), the question likely contains a typo where 'C: Muck' should have been included in the 'Both' option, or 'D' was intended to mean 'Both A & C'. However, strictly reading the options, neither A nor B alone covers all organic soils, and D is factually incorrect as Arid soil is not organic. Given typical MCQ patterns, 'Peat' (A) is the most common direct answer for undecomposed organic soil, but 'Muck' (C) is also correct. If forced to choose the best single answer without assuming a typo, A is standard. However, if D implies a typo for 'Both A & C', it's the intended answer. Let's look at standard definitions: Organic soils = Peat and Muck. Option D says 'Both a & b'. B is Arid. This makes D wrong. Between A and C, Peat is the more common term for organic soil in general contexts. But wait, if the question implies 'Organic soils are called...', both Peat and Muck are correct. If D is 'Both a & c', then D is right. As written, D is wrong. I will suggest A as the most likely intended single answer or note the error. Actually, in many Pakistani exam contexts, 'Peat' is the standard answer for organic soil. Let's stick with A being the best single option if D is strictly interpreted. However, often these questions have typos. If I must pick one, A is safer. But wait, if the current answer is D, and D is factually wrong, I should correct it. Is there a context where Arid soil is organic? No. So D is wrong. Between A and C, Peat is more widely recognized as 'organic soil' in general geography/agriculture quizzes. I will suggest A.