Reduction involves :

Options

A. gain of electrons
B. addition of oxygen
C. increases in oxidation number
D. loss of electrons .

Answer

### Step-by-Step Solution:

1. **Understanding Reduction**:
- Reduction is a chemical process that involves a change in the oxidation state of a substance. Specifically, it is characterized by a decrease in oxidation state.

2. **Gain of Electrons**:
- During reduction, a substance gains electrons. This is a key feature of the reduction process. For example, if a species gains electrons, its oxidation state decreases.

3. **Oxidation State Changes**:
- To illustrate this, consider the reaction where nitrogen in ammonia (NH3) is converted to nitrogen gas (N2). In NH3, nitrogen has an oxidation state of -3, while in N2, it has an oxidation state of 0. This increase in oxidation state indicates that nitrogen has been oxidized, while the species that gained electrons (like bromine in this case) has been reduced.

4. **Electrochemical Context**:
- In electrochemical cells, reduction occurs at the cathode, where the species that gains electrons is reduced. This is important for understanding how reduction and oxidation occur in electrochemical reactions.

5. **Example Analysis**:
- In the reaction NH3 + 3Br2 → N2 + 6HBr, bromine (Br2) starts with an oxidation state of 0 and ends with an oxidation state of -1 in HBr. This indicates that bromine has gained electrons, confirming that it is undergoing reduction.

6. **Conclusion**:
- Therefore, reduction can be summarized as a process that involves the gain of electrons, a decrease in oxidation state, and occurs at the cathode in electrochemical cells.