Which of the following represents the correct decreasing order of acidic strength of following ?
(i)Methanoic acid
(ii)Ethanoic acid
(iii)Propanoic acid
(iv)Butanoic acid

Options

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

Explanation

To determine the correct decreasing order of acidic strength among methanoic acid, ethanoic acid, propanoic acid, and butanoic acid, we need to analyze the factors that influence the acidity of carboxylic acids.

### Step-by-Step Solution:

1. **Identify the Acids**:
- Methanoic acid (HCOOH)
- Ethanoic acid (CH3COOH)
- Propanoic acid (C2H5COOH)
- Butanoic acid (C3H7COOH)

2. **Understand Acidic Strength**:
- The acidity of carboxylic acids is primarily determined by the stability of the carboxylate ion formed after deprotonation.
- The more stable the carboxylate ion, the stronger the acid.

3. **Consider the Inductive Effect**:
- The alkyl groups (CH3, C2H5, C3H7, C4H9) are electron-donating groups and exhibit a +I (positive inductive) effect.
- This +I effect destabilizes the carboxylate ion, thus decreasing the acidity of the acid.

4. **Order the Alkyl Groups**:
- The order of +I effect strength from weakest to strongest is:
- CH3 (methyl) < C2H5 (ethyl) < C3H7 (propyl) < C4H9 (butyl)
- Therefore, as we move from methanoic acid to butanoic acid, the +I effect increases, leading to a decrease in acidity.

5. **Determine the Order of Acidity**:
- Methanoic acid has no alkyl group to exert a +I effect, making it the strongest acid.
- Ethanoic acid has one alkyl group (CH3), which slightly decreases its acidity compared to methanoic acid.
- Propanoic acid has two alkyl groups (C2H5), further decreasing its acidity.
- Butanoic acid, with three alkyl groups (C3H7), is the least acidic.

6. **Final Order of Acidity**:
- The correct decreasing order of acidic strength is:
- Methanoic acid > Ethanoic acid > Propanoic acid > Butanoic acid

### Conclusion:
Thus, the correct answer is:
**Methanoic acid > Ethanoic acid > Propanoic acid > Butanoic acid**.