ANS:D - Oxygen

Among the options provided, Oxygen does not come under the category of 'secondary nutrient' for plant growth. Secondary nutrients are essential elements required by plants in relatively smaller quantities compared to primary nutrients (nitrogen, phosphorus, and potassium), but they are still vital for healthy plant growth. Calcium, magnesium, and sulfur are considered secondary nutrients because they are needed by plants in moderate amounts for various physiological processes. Oxygen, on the other hand, is not typically classified as a nutrient for plants because it is abundant in the atmosphere and readily available to plants through the process of photosynthesis. While oxygen is essential for plant respiration and energy production, it is not considered a nutrient in the same way as elements like nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur, which are actively taken up by plants from the soil or other sources. Therefore, oxygen does not fall under the category of 'secondary nutrient' for plant growth.