Lewis base is

Options

A. `CO_(2)`
B. `SO_(3)`
C. `SO_(2)`
D. ROH

Explanation

### Step-by-Step Solution:

1. **Definition of Lewis Base**: A Lewis base is defined as a substance that can donate a pair of electrons to form a covalent bond. This definition is part of the broader Lewis theory of acids and bases, where Lewis acids accept electron pairs and Lewis bases donate them.

2. **Example of a Lewis Base**: In the video, the example given is ROH (where R represents an alkyl group and OH represents a hydroxyl group). The oxygen atom in the hydroxyl group has two lone pairs of electrons.

3. **Electron Donation**: The oxygen in the hydroxyl group can donate one of its lone pairs to form a bond with a Lewis acid. This ability to donate electrons is what classifies it as a Lewis base.

4. **Comparison with CO2**: The video contrasts CO2 with the Lewis base example. CO2 does not donate electrons because it has a complete octet and does not have available lone pairs to share. Instead, CO2 is classified as a Lewis acid due to its ability to accept electron pairs.

5. **Conclusion**: Therefore, a Lewis base is any species that can donate a pair of electrons, and in the example provided, the hydroxyl group (OH) in ROH acts as a Lewis base.

### Final Answer:
A Lewis base is a substance that can donate a pair of electrons to form a covalent bond. An example is the hydroxyl group (OH) in alcohols (ROH), where the oxygen atom can donate its lone pair.

---