There is a narrow range of concentration at which the elements are optimum. This statement is concerned with :-

Options

A. Tracer elements
B. Small sized elements
C. Micronutrients
D. Balancing elements

Answer

To solve the question "There is a narrow range of concentration at which the elements are optimum. This statement is concerned with:", we will analyze the options provided and determine which one correctly relates to the concept of optimum concentration.

### Step-by-Step Solution:

1. **Understanding the Statement**:
- The statement refers to a specific range of concentration for certain elements that is considered optimal for biological functions. This implies that there are elements whose effectiveness or necessity is dependent on their concentration.

2. **Analyzing the Options**:
- **Option A: Trace Elements**:
- Trace elements, also known as minor elements, are essential for various biological processes but are required in very small amounts. Their concentration must be within a narrow range to be effective, as too little or too much can lead to deficiencies or toxicity.
- **Option B: Small Sized Elements**:
- This option refers to the size of the elements and does not relate to their concentration or biological necessity.
- **Option C: Micronutrients**:
- Micronutrients are indeed required in trace amounts for normal growth and development, but the term is broader and does not specifically emphasize the narrow range of concentration as much as trace elements do.
- **Option D: Balancing Elements**:
- Balancing elements help mitigate toxicity from other elements but do not specifically relate to the concept of optimum concentration.

3. **Conclusion**:
- After analyzing all the options, **Option A: Trace Elements** is the correct answer. Trace elements are characterized by their requirement in very low concentrations and their effectiveness being dependent on maintaining an optimal range.

### Final Answer:
**The statement is concerned with: Trace Elements (Option A).**