The resonance effect is defined as 'the polarity produced in the molecule by the interaction of two `pi`-bonds or between a `pi`-bond and lone pair of electrons present on an adjacentatom.' The effect is transmitted through the chain.
In positive resonance effect, the transfer of electrons is away from an atom or substituent group attached to the conjugated system. This electron displacement makes certain positions in the molecule of high electron densities. In negative resonance effect, the transfer of electrons is towards the atom or substituent group attached to the conjugated system.
Which of the following shows +M effect ?

Options

A. `- N(CH_(3))_(2)`
B.
C. `- CN`
D. Both (a) and (c)

Explanation

To determine which of the following groups shows a positive resonance effect (+M effect), we need to analyze the given groups based on their ability to donate electron density through resonance. Here’s a step-by-step solution:

### Step 1: Understand the Positive Resonance Effect
The positive resonance effect (+M effect) occurs when a substituent group donates electron density to a conjugated system. This typically involves groups that have lone pairs of electrons that can participate in resonance with the π-system of the molecule.

**Hint:** Look for groups with lone pairs that can interact with adjacent π-bonds.

### Step 2: Analyze the Given Groups
1. **Ammonia (NH3)**:
- Ammonia has a lone pair on nitrogen. When attached to a conjugated system (like benzene), the lone pair can resonate into the π-system, donating electron density. Thus, it shows a +M effect.

2. **Carbonyl Group (C=O)**:
- The carbonyl group is an electron-withdrawing group because the oxygen is more electronegative than carbon. It pulls electron density away from the conjugated system, showing a -M effect.

3. **Cyanide Group (CN)**:
- The cyanide group also has a triple bond between carbon and nitrogen. The nitrogen is more electronegative and pulls electron density towards itself, indicating a -M effect.

### Step 3: Conclusion
From the analysis:
- **Ammonia (NH3)** shows a +M effect.
- **Carbonyl Group (C=O)** and **Cyanide Group (CN)** both show a -M effect.

Thus, the group that shows a +M effect is **Ammonia (NH3)**.

### Final Answer
**Ammonia (NH3) shows the +M effect.**

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