Concept:Thin-shelled eggs in layers usually indicate a calcium deficiency in the diet.Explanation:Calcium is essential for the formation of strong eggshells.If the feed lacks enough calcium, the hen produces eggs with weak or thin shells.Adding a calcium-rich ingredient helps correct this defect.Among the given options, oyster shell meal is a rich source of calcium.Maize, soya bean meal, and cowpea are not primary calcium supplements.Answer:D. Oyster shell meal