Which of the following is a redox reactions:

Options

A. `NaCl + KNO_(3) to NaNO_(3) + KCl`
B. `CaC_2O_4 + 2HCl to CaCl_2 + H_2C_2O_4`
C. `Mg(OH)_2 + 2NH_4Cl to MgCl_2 + 2NH_4OH`
D. `Zn + 2AgCN to 2Ag + Zn(CN)2`

Explanation

To determine which of the given reactions is a redox reaction, we need to analyze each option to see if there are changes in the oxidation states of the elements involved. A redox reaction involves both oxidation (loss of electrons) and reduction (gain of electrons).

### Step-by-Step Solution:

1. **Understand Redox Reactions**:
- A redox reaction involves both oxidation and reduction processes. Oxidation is the loss of electrons (increase in oxidation state), while reduction is the gain of electrons (decrease in oxidation state).

2. **Analyze Option A: NaCl + KNO3 → NaNO3 + KCl**:
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