Hardening of oils is caused by

Options

A. `H_(2)`
B. `N_2`
C. `O_2`
D. `CO_2`

Explanation

### Step-by-Step Solution:

1. **Understanding the Process**: The hardening of oils refers to the conversion of liquid oils (which are usually unsaturated fats) into solid or semi-solid fats (saturated fats). This process is known as hydrogenation.

2. **Identifying Unsaturated Oils**: Oils are typically unsaturated, meaning they contain double bonds between carbon atoms in their fatty acid chains. These double bonds make them liquid at room temperature.

3. **Role of Hydrogen**: The hardening process involves the addition of hydrogen (H2) to the unsaturated oils. This addition of hydrogen atoms breaks the double bonds between carbon atoms.

4. **Catalyst Requirement**: The hydrogenation reaction usually requires a catalyst to proceed efficiently. Nickel (Ni) is commonly used as a catalyst in this process.

5. **Chemical Reaction**: When unsaturated oils react with hydrogen in the presence of nickel, the double bonds are saturated with hydrogen atoms, resulting in the formation of saturated fats. This reaction can be represented as:
\[ \text{Unsaturated Oil} + H_2 \xrightarrow{\text{Ni}} \text{Saturated Fat} \]

6. **Conclusion**: Therefore, the hardening of oils is caused by the addition of hydrogen (H2) through the process of hydrogenation, making the correct answer to the question "Hardening of oils is caused by" is Hydrogen (H2).