(a) Moderate decrease of increase on micronutrients causes deficiency and toxicaity symptoms respectively.
(b) Excess of manganese causes toxicity of iron calcium and molybdenum.
(c ) A macronutrients said to be toxic when present below a critical concentration.

Options

A. Only (a) is correct
B. Only (b) is correct
C. (b) & (c ) are correct
D. (a) & (c ) are correct

Answer

To analyze the statements provided in the question, we will evaluate each one step by step.

### Step 1: Evaluate Statement (a)
**Statement:** Moderate decrease of increase on micronutrients causes deficiency and toxicity symptoms respectively.

**Explanation:** Micronutrients are essential elements required in trace amounts by plants. A slight decrease in their concentration can lead to deficiency symptoms, while a slight increase can cause toxicity symptoms. This is because plants have a narrow range of optimal concentrations for micronutrients. Therefore, this statement is **correct**.

### Step 2: Evaluate Statement (b)
**Statement:** Excess of manganese causes toxicity of iron, calcium, and molybdenum.

**Explanation:** Excess manganese can indeed lead to the inhibition of the uptake of other essential nutrients, particularly iron and calcium. However, molybdenum is not typically affected in the same way. Instead, magnesium is more relevant in this context. Therefore, this statement is **incorrect**.

### Step 3: Evaluate Statement (c)
**Statement:** A macronutrient is said to be toxic when present below a critical concentration.

**Explanation:** Macronutrients are required in larger quantities, and they do not typically exhibit toxicity when present below a certain concentration. Instead, deficiency symptoms may occur. Therefore, this statement is **incorrect**.

### Conclusion
Based on the evaluations:
- Statement (a) is correct.
- Statement (b) is incorrect.
- Statement (c) is incorrect.

Thus, the correct answer is that only statement (a) is true.

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