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. (a) (i) `gt` (ii) `gt` (iii) `gt` (iv)
B. (b) (ii) `gt` (iii) `gt` (iv) `gt` (i)
C. (c) (i) `gt` (iv) `gt` (iii) `gt` (ii)
D. (d) (iv) `gt` (i) `gt` (iii) `gt` (ii)

Explanation

To determine the correct decreasing order of acidic strength among methanoic acid (HCOOH), ethanoic acid (CH3COOH), propanoic acid (CH3CH2COOH), and butanoic acid (CH3CH2CH2COOH), we need to analyze the effects of the alkyl groups on the acidity of these carboxylic acids.

### Step-by-Step Solution:

1. **Identify the Acids and Their Structures**:
- Methanoic acid (HCOOH)
- Ethanoic acid (CH3COOH)
- Propanoic acid (CH3CH2COOH)
- Butanoic acid (CH3CH2CH2COOH)

2. **Understand the Inductive Effect**:
- Alkyl groups are electron-donating due to their +I (positive inductive) effect. This means they push electron density towards the carboxyl group (-COOH).
- The presence of electron-donating groups decreases the acidity because they make it harder for the hydrogen ion (H+) to dissociate from the carboxylic acid.

3. **Analyze the Alkyl Groups**:
- Methanoic acid has no alkyl group (only hydrogen), which means it has the least +I effect.
- Ethanoic acid has one methyl group (CH3), which has a moderate +I effect.
- Propanoic acid has two carbon atoms (CH3CH2), which increases the +I effect compared to ethanoic acid.
- Butanoic acid has three carbon atoms (CH3CH2CH2), leading to the highest +I effect among the four acids.

4. **Determine the Order of Acidity**:
- Since the acidic strength is inversely proportional to the +I effect, the order of acidity will be:
- Methanoic acid (HCOOH) > Ethanoic acid (CH3COOH) > Propanoic acid (CH3CH2COOH) > Butanoic acid (CH3CH2CH2COOH)

5. **Final Order**:
- The correct decreasing order of acidic strength is:
- HCOOH > CH3COOH > CH3CH2COOH > CH3CH2CH2COOH

### Conclusion:
The correct answer is option A: 1 > 2 > 3 > 4.