Essential elements are -

Options

A. not required for normal reproduction.
B. not replaced by other elements.
C. indirectly involved in metabolism.
D. required only in those metabolic process which lead to increase in cytoplasmic mass.

Answer

**Step-by-Step Solution:**

1. **Definition of Essential Elements**: Essential elements are nutrients that are required for the normal growth and development of plants. They cannot be replaced by other elements, and they play specific roles in various physiological processes.

2. **Classification of Essential Elements**: There are 17 essential elements identified in plants. These are classified into two categories:
- **Macronutrients**: There are 9 macronutrients that are required in larger quantities. These include elements like nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S).
- **Micronutrients**: There are 8 micronutrients that are required in smaller amounts. These include elements like iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), boron (B), chlorine (Cl), and nickel (Ni).

3. **Characteristics of Essential Elements**:
- **Irreplaceability**: Essential elements cannot be substituted by other elements. For example, if a plant requires iron for a specific function, providing copper will not fulfill that requirement.
- **Specificity**: Each essential element has a specific role in plant metabolism. For instance, iron is crucial for chlorophyll synthesis, while nitrogen is vital for protein synthesis.

4. **Analysis of Options**:
- **Option 1**: "Not required for normal reproduction" - This is incorrect because essential elements are indeed required for reproduction.
- **Option 2**: "Not replaceable by other elements" - This is correct as essential elements cannot be substituted.
- **Option 3**: "Indirectly involved in metabolism" - This is incorrect; essential elements are directly involved in metabolic processes.
- **Option 4**: "Required only in those metabolic processes that need to increase in cytoplasmic masses" - This is also incorrect as essential elements are involved in various metabolic processes, not just those related to cytoplasmic mass.

5. **Conclusion**: The correct answer is **Option 2**: Essential elements are not replaceable by other elements.

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