Introduction:
Mixed fertilizers are fertilizers that contain a combination of two or more primary nutrients required by plants for their growth and development. These primary nutrients include nitrogen (N), phosphorus (P), and potassium (K). Mixed fertilizers are commonly used in agriculture to improve soil fertility and enhance crop yields.

Explanation:
Among the given options, option 'D' (NPK) is a mixed fertilizer. Let's understand why:

NPK:
NPK stands for nitrogen (N), phosphorus (P), and potassium (K). These three elements are crucial for plant growth and are commonly referred to as primary macronutrients.

- Nitrogen (N): Nitrogen is essential for the formation of proteins, enzymes, and chlorophyll in plants. It plays a vital role in promoting leaf and stem growth.
- Phosphorus (P): Phosphorus is necessary for root development, flowering, and fruiting in plants. It is also involved in energy transfer and storage.
- Potassium (K): Potassium helps in regulating water balance, improving disease resistance, and enhancing overall plant vigor.

Importance of NPK Fertilizers:
NPK fertilizers provide a balanced combination of these essential nutrients, ensuring that plants receive all the necessary elements for healthy growth. They are widely used in agriculture to overcome nutrient deficiencies in soil and promote optimal crop production.

Other Options:
Let's briefly discuss the other options:

a) Urea: Urea is a nitrogenous fertilizer that contains only nitrogen. It is not a mixed fertilizer as it lacks phosphorus and potassium.

b) CAM: CAM is not a fertilizer; it stands for Crassulacean Acid Metabolism, which is a photosynthetic adaptation in plants.

c) Ammonium Sulphate: Ammonium sulfate is also a nitrogenous fertilizer that provides nitrogen to plants. It does not contain phosphorus and potassium, so it is not a mixed fertilizer.

Conclusion:
Among the given options, only NPK (option D) is a mixed fertilizer as it contains a combination of nitrogen, phosphorus, and potassium. The other options either contain a single nutrient or are not fertilizers.