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Explanation

Solution: Permanent magnets must be made from materials that retain magnetism for long periods. High carbon steel is a good base because it can be hardened and magnetized effectively. Adding cobalt to high carbon steel improves its magnetic strength and stability by increasing the material’s ability to resist demagnetization. For conventional permanent magnets, the optimal composition includes around 15% cobalt alloyed with high carbon steel. Higher percentages, like 35%, are not standard in steel-based magnets and are more applicable to specialized magnet alloys such as Alnico, but even then, typical cobalt content does not reach 35%. Therefore, Option A (15% of cobalt) is the correct choice for permanent magnets made from high carbon steel.