The correct answer is option 2, which is Magnesium.

Micro-nutrients are essential elements required by plants in small amounts for their growth and development. These nutrients include iron, molybdenum, and manganese. However, magnesium is not classified as a micro-nutrient but as a macro-nutrient.

Macro-nutrients, including elements like nitrogen, phosphorus, and potassium, are required by plants in larger quantities compared to micro-nutrients. Magnesium is considered a macro-nutrient because it is needed in relatively larger amounts for vital plant processes like photosynthesis and the production of chlorophyll.

It is important to note that while magnesium is not a micro-nutrient, it is still crucial for the healthy growth and functioning of plants. Magnesium deficiency can lead to various problems such as yellowing of leaves and reduced plant vigor. Therefore, although not a micro-nutrient, magnesium is still essential for the overall nutrient balance and well-being of plants.