The decreasing order of -I effect of the following is
I. `-overset(o+)NH_(3)`.
II. `-NO_(2)`
III. `-CN`
IV. `-COOH`

Options

A. I gt II gt III gt IV
B. II gt I gt III gt IV
C. I gt II gt III gt IV
D. II gt I gt IV gt III

Answer

To determine the decreasing order of the -I (negative inductive) effect of the given groups, we need to analyze the electronegativity and the ability of each group to withdraw electron density from the rest of the molecule. The groups we have are:

1. `-NH3^+`
2. `-NO2`
3. `-CN`
4. `-COOH`

### Step 1: Analyze the -I effect of each group

1. **`-NH3^+` (Positive Ammonium Ion)**:
- The nitrogen in `NH3^+` has a positive charge, which significantly increases its electronegativity. This makes it a strong electron-withdrawing group due to the positive charge that pulls electron density towards itself. Therefore, it exhibits a very strong -I effect.

2. **`-NO2` (Nitro Group)**:
- The nitro group is known for its strong -I effect due to the presence of highly electronegative oxygen atoms. The nitro group can stabilize negative charge through resonance and inductive effects, pulling electron density towards itself effectively. Thus, it has a strong -I effect, but not as strong as `-NH3^+`.

3. **`-CN` (Cyano Group)**:
- The cyano group has a carbon triple-bonded to nitrogen. The nitrogen is electronegative and pulls electron density towards itself, but it is not as strong as the nitro group. Therefore, it has a moderate -I effect.

4. **`-COOH` (Carboxylic Acid Group)**:
- The carboxylic acid group has both a carbonyl and a hydroxyl group. While it can withdraw electrons due to the electronegative oxygen atoms, it also has lone pairs on the hydroxyl oxygen that can donate electron density, which reduces its overall -I effect compared to the other groups. Thus, it has the weakest -I effect among the four.

### Step 2: Arrange the groups in decreasing order of -I effect

Based on the analysis above, we can arrange the groups in decreasing order of their -I effect:

1. `-NH3^+` (strongest -I effect)
2. `-NO2`
3. `-CN`
4. `-COOH` (weakest -I effect)

### Final Answer
The decreasing order of -I effect is:
**`-NH3^+ > -NO2 > -CN > -COOH`**