What is the strongest acid among the given compounds? 

m-CH₃C₆H₄COOH

The correct answer is option 3. m-CH₃C₆H₄COOH.

Let us go through the analysis in detail, to understand why \(m-CH_3C_6H_4COOH\) (m-Toluic acid) is the strongest acid among the given compounds.

1. Benzylamine \((C_6H_5CH_2NH_2)\)

It has an amine group \((NH_2)\) attached to a benzyl group. Benzylamine is a base, not an acid, because the amino group \((-NH_2)\) can accept a proton \((H^+)\). The acidity of the compound will be very low; it does not donate protons easily. We can rule this out as the strongest acid.

2. Benzoic Acid \((C_6H_5COOH)\)

A carboxyl group \((-COOH)\) directly attached to a phenyl ring \((C_6H_5)\). The phenyl group is slightly electron-withdrawing due to its resonance effect. This effect stabilizes the conjugate base (benzoate ion, \(C_6H_5COO^−)\) by delocalizing the negative charge over the aromatic ring.  Benzoic acid is a relatively strong acid among carboxylic acids due to the stabilizing effect of the phenyl ring.

3. m-Toluic Acid \((m-CH_3C_6H_4COOH)\)

It has a carboxyl group \((-COOH)\) attached to a phenyl ring with a methyl group \((CH_3)\) at the meta position relative to the carboxyl group. The methyl group is an electron-donating group through the inductive effect. However, at the meta position, its inductive effect on the carboxyl group is minimal and does not significantly destabilize the conjugate base (meta-methylbenzoate ion, \(m-CH_3C_6H-4COO−)\). The meta-positioned methyl group has little influence on the acidity, so m-Toluic acid remains quite acidic, comparable to or even slightly stronger than benzoic acid.

4. p-Toluic Acid \((p-CH_3C_6H_4COOH)\):

It has a carboxyl group \((-COOH)\) attached to a phenyl ring with a methyl group \((CH_3)\) at the para position relative to the carboxyl group. The methyl group at the para position is an electron-donating group through the inductive effect. The para position allows for more effective donation of electron density through the ring to the carboxyl group, slightly destabilizing the conjugate base (para-methylbenzoate ion, \(p-CH_3C_6H_4COO^−\)). The para-positioned methyl group decreases the acidity of the carboxyl group more than the meta-positioned methyl group does, making p-Toluic acid less acidic than both benzoic acid and m-Toluic acid.

Given these points, the correct order of acidity from strongest to the weakest acid is: \(m-CH_3C_6H_4COOH > C_6H_5COOH > p-CH_3C_6H_4COOH > C_6H_5CH_2NH_2\)

Thus, the strongest acid among the given compounds is \(m-CH_3C_6H_4COOH\) (m-Toluic acid)