Reactivity of `EtMgBr` with the following in the decreasing order is :
(i) `HCHO`
(ii) `MeCHO`
(iii) `MeCOMe`
(iv) `Cl_3 C-CHO`.

Options

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

Explanation

To determine the reactivity of ethyl magnesium bromide (EtMgBr) with the given compounds in decreasing order, we need to consider the nature of the carbonyl compounds and how they interact with Grignard reagents.

### Step-by-Step Solution:

1. **Identify the Nature of the Reactants**:
- **EtMgBr** is a Grignard reagent, which acts as a strong nucleophile.
- The compounds to react with are:
- (i) HCHO (formaldehyde)
- (ii) MeCHO (acetaldehyde)
- (iii) MeCOMe (acetone)
- (iv) Cl₃C-CHO (trichloroacetaldehyde)

2. **Understand the Reactivity of Carbonyl Compounds**:
- Aldehydes are generally more reactive than ketones because they have one alkyl group (which is electron-donating) compared to two in ketones.
- Electron-withdrawing groups (like Cl) increase the electrophilicity of the carbonyl carbon, making it more reactive towards nucleophiles.

3. **Analyze Each Compound**:
- **HCHO (formaldehyde)**: No electron-donating groups, making it highly reactive.
- **MeCHO (acetaldehyde)**: Has one methyl group (electron-donating), less reactive than formaldehyde.
- **MeCOMe (acetone)**: Has two methyl groups (electron-donating), making it the least reactive among aldehydes.
- **Cl₃C-CHO (trichloroacetaldehyde)**: The presence of three chlorine atoms (electron-withdrawing) significantly increases its reactivity.

4. **Rank the Reactivity**:
- The order of reactivity based on the analysis:
1. **Cl₃C-CHO** (most reactive due to strong electron-withdrawing effect)
2. **HCHO** (high reactivity as an aldehyde)
3. **MeCHO** (less reactive due to one electron-donating group)
4. **MeCOMe** (least reactive due to two electron-donating groups)

5. **Final Order**:
- The decreasing order of reactivity of EtMgBr with the given compounds is:
- Cl₃C-CHO > HCHO > MeCHO > MeCOMe

### Conclusion:
The final order of reactivity of EtMgBr with the given compounds in decreasing order is:
- (iv) Cl₃C-CHO > (i) HCHO > (ii) MeCHO > (iii) MeCOMe.