Explanation

Magnesium is a macronutrient, not a micronutrient: plants need it in relatively large quantities, and it forms the central atom of the chlorophyll molecule. Iron, manganese and chlorine are all required only in trace amounts, which is the defining feature of micronutrients. Iron is essential for chlorophyll synthesis and electron transport, manganese for the oxygen-evolving complex in photosynthesis, and chlorine for osmosis and ionic balance. Since magnesium is needed in bulk quantities, it is the odd one out. Hence option C is correct.