One of the defining characteristics of an essential element is that it

Options

A. is only necessary for early growth of the seedling
B. Can be replaced or not replaced by another element
C. Has a direct function in the plant
D. Is found in relatively high concentration in the environment

Answer

**Step-by-Step Solution:**

1. **Understanding Essential Elements**: Essential elements are nutrients that are required by plants for their growth and development. They play crucial roles in various physiological processes.

2. **Evaluating Option 1**: The first option states that an essential element is "only necessary for early growth of the seedling." This is incorrect because essential elements are required throughout the entire life cycle of the plant, not just in the early stages.

3. **Evaluating Option 2**: The second option suggests that an essential element "can be replaced or not replaced by another element." This is also incorrect. Essential elements are non-replaceable; if a plant requires a specific element (like zinc), it cannot substitute it with another element (like copper) for the same function.

4. **Evaluating Option 3**: The third option claims that an essential element "has a direct function in the plant." This statement is correct. Essential elements are directly involved in the plant's metabolism and are necessary for various biochemical processes.

5. **Evaluating Option 4**: The fourth option states that an essential element "is found in relatively high concentration in the environment." This is incorrect because not all essential elements are abundant in the environment. For example, while nitrogen is plentiful, other essential elements like carbon may be present in lower concentrations.

6. **Conclusion**: After evaluating all options, the correct answer is option 3: "Has a direct function in the plant."