C, H, O and N are called structural elements

Options

A. They are essential to all plants
B. They are required in all biochemical reactions in plants
C. They are invariably present in all biomolecules
D. They associate with prosthetic group of enzymes, catalysing synthesis of biomolecules.

Answer

### Step-by-Step Solution:

1. **Identify the Elements**: The question states that carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) are structural elements.

2. **Understand Their Role**: These elements are fundamental components of all biomolecules. Biomolecules include carbohydrates, proteins, lipids, and nucleic acids, which are essential for life.

3. **Presence in Living Organisms**: C, H, O, and N make up about 98 to 99 percent of the mass of living organisms. This indicates their critical role in the structure and function of biological molecules.

4. **Evaluate the Options**:
- **Option A**: "They are essential to all plants." - This is misleading as it implies exclusivity to plants.
- **Option B**: "They are present in all biomolecule reaction in plants." - This is not entirely accurate as it does not encompass their structural role.
- **Option C**: "They are invariably present in all biomolecules." - This is correct as these elements are indeed found in all biomolecules.
- **Option D**: "They are associated with prosthetic group of enzyme." - This is incorrect as it does not represent the primary structural role of these elements.

5. **Conclusion**: The correct answer is that carbon, hydrogen, oxygen, and nitrogen are invariably present in all biomolecules, making Option C the right choice.