Which of the following reactions is a redox reaction?
(a).`CuSO_(4)+4NH_(3) to [Cu(NH_(3))_(4)]SO_(4)`
(b).`Na_(2)SO_(4)+BaCl_(2) to BaSO_(4)+2NaCl`
(c ).`SO_(2)+H_(2)O to H_(2)SO_(4)`
(d).`2CuSO_(4)+4KI to Cu_(2)I_(2)+2K_(2)SO_(4)+I_(2)`

Explanation

To determine which of the given reactions is a redox reaction, we need to analyze the oxidation states of the elements in each reaction and check if there is a change in oxidation states, indicating that oxidation and reduction processes are occurring.

### Step-by-Step Solution:

1. **Analyze Option (a)**:
- Reaction: `CuSO4 + 4NH3 → [Cu(NH3)4]SO4`
- Oxidation states:
- Cu in CuSO4: +2
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