[Which of the following is not an intermolecular redox reaction?

Options

A. `MgCO_(3)rarrMgO+CO_(2)`
B. `O_(2)+2H_(2)rarr2H_(2)O`
C. `K+H_(2)OrarrKOH+(1//2)H_(2)`
D. `MnBr_(3)rarrMnBr_(2)+(1//2)Br_(2)`

Explanation

To determine which of the given reactions is not an intermolecular redox reaction, we need to analyze the oxidation states of the elements involved in each reaction. A redox reaction involves the transfer of electrons, which is indicated by a change in oxidation states.

### Step-by-Step Solution:

1. **Identify the Reactions**: We need to look at the provided reactions and identify the oxidation states of the elements involved.

2. **Analyze Reaction A (Mg to MgO)**:
- Oxidation state of Mg in elemental form = 0.
- Oxidation state of Mg in MgO = +2.
- Since the oxidation state changes from 0 to +2, this is a redox reaction.

3. **Analyze Reaction B (H2 to H2O)**:
- Oxidation state of H in H2 = 0.
- Oxidation state of O in H2O = -2.
- Oxidation state of H in H2O = +1.
- Here, H is oxidized (0 to +1) and O is reduced (0 to -2). This is a redox reaction.

4. **Analyze Reaction C (K to K+ and H2)**:
- Oxidation state of K in elemental form = 0.
- Oxidation state of K in K+ = +1.
- Oxidation state of H in H2 = 0.
- H in H2O has an oxidation state of +1.
- K is oxidized (0 to +1) and H is reduced (0 to +1). This is a redox reaction.

5. **Analyze Reaction D (MnBr3)**:
- Oxidation state of Mn in MnBr3 = +3.
- Oxidation state of Mn in MnBr2 = +2.
- Oxidation state of Br in Br2 = 0.
- Mn is reduced (from +3 to +2) and Br is oxidized (0 to -1). This is a redox reaction.

6. **Conclusion**: After analyzing all the reactions, we find that Reaction A (Mg to MgO) does not involve a change in oxidation state of Mg when considering the formation of MgO from elemental Mg. Therefore, it is not an intermolecular redox reaction.

### Final Answer:
The reaction that is not an intermolecular redox reaction is **Option A (Mg to MgO)**.