Which of the following statements is correct regarding redox reactions-

Options

A. An increase in oxidation number of an element is called reduction
B. A decrease in oxidation number of an element is called oxidation.
C. A reagent which lowers the oxidation number of an element in a given substance is reductant.
D. A reagent which increases the oxidation number of an element in a given substance is reductant .

Explanation

To solve the question regarding which statement is correct about redox reactions, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Redox Reactions**:
- Redox reactions involve the transfer of electrons between two species. One species undergoes oxidation (loss of electrons, increase in oxidation number) and the other undergoes reduction (gain of electrons, decrease in oxidation number).

2. **Analyzing Option A**:
- **Statement**: "An increase in oxidation number of an element is called reduction."
- **Analysis**: This statement is incorrect. An increase in oxidation number indicates oxidation, not reduction. Therefore, Option A is false.

3. **Analyzing Option B**:
- **Statement**: "A decrease in oxidation number of an element is called oxidation."
- **Analysis**: This statement is also incorrect. A decrease in oxidation number indicates reduction, not oxidation. Therefore, Option B is false.

4. **Analyzing Option C**:
- **Statement**: "A reagent which lowers the oxidation number of an element in a given substance is a reductant."
- **Analysis**: This statement is misleading. While a reductant does indeed lower the oxidation number of another substance (by donating electrons), the reductant itself undergoes oxidation (its oxidation number increases). Hence, this statement is incorrect as it does not accurately describe the nature of a reductant. Therefore, Option C is false.

5. **Analyzing Option D**:
- **Statement**: "A reagent which increases the oxidation number of an element in a given substance is a reductant."
- **Analysis**: This statement is correct. A reductant (or reducing agent) donates electrons, causing its own oxidation number to increase while reducing another species. Therefore, Option D is true.

### Conclusion:
The correct statement regarding redox reactions is **Option D**.

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