The correct answer is option 3, which states that the three primary soil micro-nutrients are nitrogen, phosphorus, and potassium.

Option 1 is incorrect. Carbon, oxygen, and water are not considered micro-nutrients for plants. Carbon and oxygen are elements essential for all living organisms, but they are obtained from the air and not from the soil. Water is also vital for plant growth but is considered a macronutrient, not a micro-nutrient.

Option 2 is incorrect. Copper and cadmium are trace elements that can be found in the soil but are not considered primary micro-nutrients. Carbon, as mentioned earlier, is not a micro-nutrient.

Option 4 is also incorrect. Boron, zinc, and magnesium are important micro-nutrients for plants, but they are not considered primary micro-nutrients. Nitrogen, phosphorus, and potassium are considered primary micro-nutrients because they are required in larger quantities by plants compared to other micro-nutrients.

In summary, the correct answer is option 3, which correctly identifies nitrogen, phosphorus, and potassium as the three primary soil micro-nutrients.