which of the following is a 'threo' isomer ?

Options

A.
B.
C.
D.

Explanation

To determine which of the given compounds is a 'threo' isomer, we need to understand the definitions of 'threo' and 'erythro' isomers.

### Step-by-Step Solution:

1. **Understanding Isomerism**:
- Isomers are compounds that have the same molecular formula but different structural arrangements.
- There are two types of stereoisomers: enantiomers and diastereomers. The 'threo' and 'erythro' designations are used for diastereomers that differ in the arrangement of groups around a stereocenter.

2. **Defining Erythro and Threo**:
- **Erythro Isomer**: This isomer has similar groups on the same side of the molecule.
- **Threo Isomer**: This isomer has similar groups on opposite sides of the molecule.

3. **Analyzing the Compounds**:
- To identify which compound is a 'threo' isomer, we need to look at the arrangement of the substituents around the stereocenters in the given options.
- We will evaluate each option based on the arrangement of the groups.

4. **Identifying the Threo Isomer**:
- From the video transcript, it is mentioned that the compound labeled as option B (which has CH3, Br, H, and OH groups) forms a threo isomer.
- This means that in option B, the similar groups (in this case, the hydrogen atoms) are positioned on opposite sides of the molecule.

5. **Conclusion**:
- Therefore, the compound that is a 'threo' isomer is option B.