Tracer elements are

Options

A. microelements
B. macroelements
C. radioisotopes
D. vitamins

Answer

**Step-by-Step Solution:**

1. **Understanding Tracer Elements**: Tracer elements, also known as micronutrients, are essential elements that plants require in very small quantities. They play crucial roles in various physiological processes.

2. **Identifying the Options**: The question provides four options:
- a. microelements
- b. macroelements
- c. radioisotopes
- d. vitamins

3. **Analyzing Each Option**:
- **Microelements**: These are elements required in trace amounts (less than 10 millimoles per kg of dry matter), which aligns with the definition of tracer elements.
- **Macroelements**: These are required in larger quantities and do not fit the definition of tracer elements.
- **Radioisotopes**: These are unstable isotopes that emit radiation and are not related to plant nutrition in the context of essential nutrients.
- **Vitamins**: While important for various biological functions, they are not classified as mineral nutrients and do not fit the definition of tracer elements.

4. **Conclusion**: Based on the analysis, the correct answer is option A, microelements, as they are synonymous with tracer elements in the context of plant nutrition.

**Final Answer**: a. microelements

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