**Step-by-Step Solution:**
1. **Identify the Elements:**
The elements given in the question are Nitrogen (N), Phosphorus (P), Potassium (K), Sodium (Na), Silicon (Si), and Cobalt (Co).
2. **Classify the Elements:**
- **Macronutrients:** Nitrogen (N), Phosphorus (P), and Potassium (K) are classified as macronutrients because they are required by plants in larger quantities (more than 10 millimoles per kg of dry matter).
- **Beneficial Elements:** Sodium (Na), Silicon (Si), and Cobalt (Co) are classified as beneficial elements. These elements are not essential for the completion of the plant life cycle, meaning that plants can survive without them, but they can enhance growth and yield when present.
3. **Understand the Functions:**
- **Nitrogen (N):** Essential for the synthesis of proteins, nucleic acids, and vitamins.
- **Phosphorus (P):** Important for the formation of cell membranes, nucleic acids, and energy transfer (ATP).
- **Potassium (K):** Plays a key role in regulating stomatal opening and closing, and maintaining cell turgidity.
- **Sodium (Na), Silicon (Si), and Cobalt (Co):** While they can enhance plant growth and development, they are not critical for survival.
4. **Conclusion:**
Based on the classification and functions of the elements, we conclude that there are 3 essential elements (N, P, K) and 3 beneficial elements (Na, Si, Co).
5. **Select the Correct Option:**
The correct answer to the question is option C, which states that there are 3 essential and 3 beneficial elements.
---
