Among `-OH and gt C=0 `, the group causing + M-effect is .......... group.

Explanation

To determine which group causes a +M (positive mesomeric) effect between -OH (hydroxyl) and C=O (carbonyl), we can analyze the behavior of these functional groups in a conjugated system.

### Step-by-Step Solution:

1. **Understanding Mesomeric Effect**: The mesomeric effect refers to the delocalization of electrons in a molecule. A +M effect indicates that a group donates electron density to the rest of the molecule, while a -M effect indicates that a group withdraws electron density.

2. **Analyzing the -OH Group**: The hydroxyl group (-OH) has a lone pair of electrons on the oxygen atom. In a conjugated system, these lone pairs can participate in resonance. For example, if we have a structure like:
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