Arrange the following groups in order of decreasing +M effect
(i) `-overset(o+)(O)` (ii) `-NH_2` (iii) `-OH` (iv) `-NHCOCH_3`

Options

A. `i gt ii gt iii gt iv `
B. `iv gt iii gt ii gt i`
C. `i gt iii gt ii gt iv`
D. `i gt iv gt iii gt ii`

Explanation

To arrange the given groups in order of decreasing +M (positive mesomeric) effect, we need to analyze each group based on its ability to donate electron density through resonance. The groups provided are:

1. `-O^+` (positively charged oxygen)
2. `-NH2` (amino group)
3. `-OH` (hydroxyl group)
4. `-NHCOCH3` (acetamido group)

### Step-by-Step Solution:

**Step 1: Understand the +M Effect**
The +M effect refers to the ability of a substituent to donate electron density to a conjugated system (like a benzene ring) through resonance. Groups that can donate lone pairs of electrons or have a less electronegative atom tend to have a stronger +M effect.

**Step 2: Analyze Each Group**
- **`-O^+`**: This group has a positive charge on oxygen, which makes it an electron-withdrawing group rather than an electron-donating group. Therefore, it has the least +M effect.

- **`-NH2`**: The amino group has a lone pair of electrons on nitrogen that can be donated to the benzene ring, making it a strong +M group. It is more effective than `-OH` due to nitrogen's lower electronegativity compared to oxygen.

- **`-OH`**: The hydroxyl group also has a lone pair of electrons that can participate in resonance. However, oxygen is more electronegative than nitrogen, which makes the +M effect of `-OH` slightly less than that of `-NH2`.

- **`-NHCOCH3`**: The acetamido group has a nitrogen atom that can donate its lone pair, but the presence of the carbonyl (C=O) adjacent to the nitrogen can have a slight withdrawing effect, making it less effective than `-NH2` but potentially more effective than `-OH`.

**Step 3: Rank the Groups**
Based on the analysis:
1. `-NH2` (strong +M effect)
2. `-NHCOCH3` (moderate +M effect)
3. `-OH` (less +M effect)
4. `-O^+` (no +M effect)

### Final Order:
The final order of the groups in decreasing +M effect is:
**`-NH2 > -NHCOCH3 > -OH > -O^+`**