Ortho isomer is the strongest one because in this case --- 1.ortho effect..2... the -I effect of the -No2 grp is the most powerful because distance from the -COOH grp is minimum ...3...the corresponding Carboxylic ion is stabilised by intramolecular electrostatic interaction between the carboxyl and the nitro group...So...the correct answer is option A..

The acidic strength of a compound is determined by the stability of the conjugate base formed when the compound loses a proton. The more stable the conjugate base, the stronger the acid. In this case, we are comparing three different isomers of nitrobenzoic acid (o-, m-, and p- isomers) and p-nitrophenol.

1. o-Nitrobenzoic acid:
- In the o-isomer, the nitro group (-NO2) is attached to the benzene ring in the ortho position, which is adjacent to the carboxylic acid (-COOH) group.
- The negative charge of the conjugate base formed after the loss of a proton is delocalized between the carboxylate group and the ortho position of the benzene ring.
- This delocalization of the negative charge stabilizes the conjugate base and increases the acidic strength of o-nitrobenzoic acid.

2. m-Nitrobenzoic acid:
- In the m-isomer, the nitro group is attached to the benzene ring in the meta position, which is not directly adjacent to the carboxylic acid group.
- The negative charge of the conjugate base formed after the loss of a proton is not delocalized as effectively as in the o-isomer.
- Therefore, m-nitrobenzoic acid is less acidic than the o-isomer.

3. p-Nitrobenzoic acid:
- In the p-isomer, the nitro group is attached to the benzene ring in the para position, which is not directly adjacent to the carboxylic acid group.
- Similar to the m-isomer, the negative charge of the conjugate base formed after the loss of a proton is not delocalized as effectively as in the o-isomer.
- Therefore, p-nitrobenzoic acid is also less acidic than the o-isomer.

4. p-Nitrophenol:
- In p-nitrophenol, the hydroxyl group (-OH) is attached to the benzene ring in the para position, and the nitro group is attached to the phenol ring.
- The presence of the hydroxyl group increases the stability of the conjugate base by resonance delocalization of the negative charge.
- Therefore, p-nitrophenol is more acidic than the p-nitrobenzoic acid isomers.

In summary, among the given compounds, o-nitrobenzoic acid has the maximum acidic strength due to the effective delocalization of the negative charge in its conjugate base.