Which of the following have `-M` effect (`overline e` - withdrawing mesomeric effect) ?

Options

A.
B. `- SO_3H`
C. `-OR`
D. `-Br`

Explanation

To determine which of the following groups exhibit the `-M` effect (electron-withdrawing mesomeric effect), we need to analyze each group based on its ability to withdraw electrons through resonance. The `-M` effect is characterized by groups that can pull electron density away from a conjugated system or double bond.

### Step-by-Step Solution:

1. **Understanding the `-M` Effect**:
- The `-M` effect refers to the ability of certain substituents to withdraw electron density from a conjugated system or double bond through resonance. This typically occurs when the substituent has a lone pair of electrons that can be delocalized into the system.

2. **Identifying Groups**:
- We need to analyze the given functional groups to see if they exhibit the `-M` effect.

3. **Analyzing the CO Group**:
- The carbonyl group (C=O) is a strong electron-withdrawing group due to the electronegativity of oxygen. It can pull electron density from adjacent double bonds or conjugated systems, thus exhibiting the `-M` effect.

4. **Analyzing the SO3H Group**:
- The sulfonic acid group (SO3H) is also an electron-withdrawing group. It has a strong `-M` effect because it can withdraw electron density from the conjugated system through resonance.

5. **Analyzing the OR Group**:
- The alkoxy group (OR, where R is an alkyl group) does not exhibit the `-M` effect. Instead, it shows a `+M` effect (electron-donating) because the oxygen can donate its lone pair into the conjugated system.

6. **Analyzing the Br Group**:
- Bromine (Br) is a halogen that also exhibits a `+M` effect due to its ability to donate electron density through resonance. Thus, it does not show the `-M` effect.

7. **Conclusion**:
- The groups that exhibit the `-M` effect are the CO group and the SO3H group. The OR and Br groups do not exhibit this effect.

### Final Answer:
The groups that have the `-M` effect are:
- CO group
- SO3H group