Many of the energy reactions in living systems are oxidation reduction (redox) reactions. Which one of the following is always transferred in a redox reaction?

Options

A. Electron
B. Proton
C. Hydrogen
D. Oxygen

Explanation

**Step-by-Step Solution:**

1. **Understanding Redox Reactions**:
- Redox reactions, short for oxidation-reduction reactions, involve the transfer of electrons between two species. This is the defining characteristic of redox reactions.

2. **Identifying the Transfer**:
- In a redox reaction, one species loses electrons (oxidation) while another gains electrons (reduction). The transfer of electrons is what changes the oxidation states of the atoms involved.

3. **Analyzing the Options**:
- The question provides four options: electron, proton, hydrogen, and oxygen.
- Among these, electrons are the particles that are always transferred in redox reactions.

4. **Conclusion**:
- Therefore, the correct answer to the question is "electron," as it is always transferred in a redox reaction.

**Final Answer**: Electron

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