Amongst `-OH, -CN, CI, -NO_2` groups, which do exert + M-effect when present in conjugation with a double bond ?

Explanation

To determine which of the groups -OH, -CN, -Cl, and -NO2 exert a +M (positive mesomeric) effect when present in conjugation with a double bond, we will analyze each group based on its ability to donate electron density through resonance.

### Step-by-Step Solution:

1. **Understanding the +M Effect**:
The +M effect refers to the ability of a substituent to donate electron density to an adjacent π system (like a double bond) through resonance. This typically occurs when the substituent has lone pairs of electrons that can be delocalized.

2. **Analyzing the -OH Group**:
...