1. Electrical conductivity in aqueous solution

For weak acids of the same concentration, electrical conductivity depends on the extent of ionization:

$\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-$

Greater ionization $\Rightarrow$ more ions in solution $\Rightarrow$ higher conductivity.

So we must compare the acid strengths of the three acids.

2. Compare acid strengths using known order

The acids are:

• Formic acid: $\text{HCOOH}$
• Acetic acid: $\text{CH}_3\text{COOH}$
• Benzoic acid: $\text{C}_6\text{H}_5\text{COOH}$

Their approximate acid strengths are:

• Formic acid is stronger than acetic acid because the methyl group in acetic acid has a $+I$ effect, which decreases acidity.
• Benzoic acid is stronger than acetic acid because the phenyl group exerts an overall electron-withdrawing effect relative to methyl.
• Formic acid is stronger than benzoic acid.

Thus, the acidity order is:

$\text{Formic acid} > \text{Benzoic acid} > \text{Acetic acid}$

That is,

$A > C > B$

3. Relate acidity to conductivity

Since all are weak acids at the same concentration ($0.1\,M$), stronger acid means larger dissociation and hence higher conductivity.

Therefore, the decreasing order of electrical conductivity is:

$A > C > B$

4. Check options

• Option A: $A > B > C$ ❌
• Option B: $C > B > A$ ❌
• Option C: $A > C > B$ ✅
• Option D: $C > A > B$ ❌

So the correct option is C.

5. Comparison with stored answer

Stored correct answer: C

My derived answer: C

They agree.