Which of the following is not a double displacement reaction?

Options

A. `BaCl_2 + H_2SO_4 to BaSO_4 + 2HCl`
B. `CuSO_4 + H_2S to CuS + H_2SO_4`
C. `NaOH + HCl to NaCl+ H_2O`
D. `ZnO + C to Zn+ CO`

Explanation

To determine which of the following reactions is not a double displacement reaction, we first need to understand what a double displacement reaction is. A double displacement reaction occurs when two compounds react and the ions in them switch places, resulting in the formation of two new compounds.

### Step-by-Step Solution:

1. **Understand Double Displacement Reactions**:
- A double displacement reaction involves two ionic compounds where the cations and anions exchange partners. The general form can be represented as:
\[ AB + CD \rightarrow AD + CB \]
- Here, A and C are cations, while B and D are anions.

2. **Analyze the Given Options**:
- **Option A**: \( \text{BaCl}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + \text{HCl} \)
- In this reaction, Ba displaces H and forms BaSO4, while HCl is formed. This is a double displacement reaction.

- **Option B**: \( \text{CuSO}_4 + \text{H}_2\text{S} \rightarrow \text{CuS} + \text{H}_2\text{SO}_4 \)
- Here, Cu displaces H and forms CuS, while H2SO4 is produced. This is also a double displacement reaction.

- **Option C**: \( \text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2 \)
- In this case, Na displaces H and forms NaCl, while H2 is produced. This is a double displacement reaction as well.

- **Option D**: \( \text{ZnO} + C \rightarrow \text{Zn} + \text{CO} \)
- Here, carbon (C) displaces zinc (Zn) from zinc oxide (ZnO) to form zinc and carbon monoxide (CO). This is a single displacement reaction, not a double displacement.

3. **Conclusion**:
- From the analysis, **Option D** is the reaction that is not a double displacement reaction. It is a single displacement reaction.

### Final Answer:
**Option D**: \( \text{ZnO} + C \rightarrow \text{Zn} + \text{CO} \) is not a double displacement reaction.