In the context of plant nutrition, nutrients are broadly classified into two groups: macronutrients and micronutrients based on the amounts needed by plants. Macronutrients are required in larger amounts while micronutrients are needed in smaller quantities.

Option 1: Manganese is classified as a micronutrient, not a macronutrient, as it`s only necessary in trace amounts for plant growth.

Option 2: Copper, like manganese, is also a micronutrient. It is essential for plant growth but is needed in very small quantities.

Option 3: Magnesium is a macronutrient. Magnesium is needed by plants in larger quantities compared to manganese and copper. It is involved in several critical metabolic processes like photosynthesis, which makes it a macronutrient.

Option 4: Chlorine is again, a micronutrient. Plants need chlorine but in minimal amounts.

So, the correct answer is indeed 3 - Magnesium is an example of a macronutrient. Each classification is based on the quantity required by plants for healthy growth and development.