which of the following is a redox reaction ?

Options

A. `NaCl+KNO_(3)rarrNaNO_(3)+KCl`
B. `CaC_(2)O_(4)+2HClrarrCaCl_(2)+H_(2)C_(2)O_(4)`
C. `Mg(OH)_(2)+2NH_(4)ClrarrMgCl_(2)+2NH_(4)OH`
D. `Zn+2AgCNrarr2Ag+Zn(CN)_(2)`

Explanation

To determine which of the following reactions is a redox reaction, we need to analyze the oxidation states of the elements involved in each reaction. A redox reaction involves the simultaneous occurrence of oxidation (increase in oxidation state) and reduction (decrease in oxidation state).

### Step-by-Step Solution:

1. **Identify Oxidation States**:
- For each species in the given reactions, we need to identify the oxidation states of the elements involved.

2. **Check for Changes in Oxidation States**:
- We will compare the oxidation states of the reactants and products to see if any element has undergone a change in oxidation state.

3. **Determine if Oxidation and Reduction Occur**:
- If one element is oxidized (its oxidation state increases) and another is reduced (its oxidation state decreases), then the reaction is a redox reaction.

4. **Analyze Each Reaction**:
- For each reaction provided:
- **First Reaction**: No change in oxidation states. Not a redox reaction.
- **Second Reaction**: No change in oxidation states. Not a redox reaction.
- **Third Reaction**: No change in oxidation states. Not a redox reaction.
- **Fourth Reaction**:
- Zinc (Zn) changes from 0 to +2 (oxidation).
- Silver (Ag) changes from +1 to 0 (reduction).
- Since one species is oxidized and another is reduced, this is a redox reaction.

5. **Conclusion**:
- The only reaction that qualifies as a redox reaction is the one involving zinc and silver.

### Final Answer:
The reaction involving zinc (Zn) and silver (Ag) is a redox reaction.

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