1. Principle of acidity

For benzoic acid derivatives, acidity increases when the substituent is electron-withdrawing, because it stabilizes the conjugate base (carboxylate ion).

Here, the substituent is $NO_2$, which is a strong electron-withdrawing group due to:

• $-I$ effect (inductive withdrawal)
• $-M$ effect (resonance withdrawal)

So all nitro-substituted benzoic acids are expected to be more acidic than benzoic acid $PhCOOH$.

Thus,

2. Compare meta and para nitrobenzoic acids

• At the para position, the $NO_2$ group exerts both $-I$ and $-M$ effects.
• At the meta position, resonance interaction with the carboxyl group is not effective for stabilizing the carboxylate through the ring in the same way, so mainly the $-I$ effect operates.

Therefore,

So,

3. Compare ortho nitrobenzoic acid

The ortho compound is the most acidic because of the ortho effect. In aromatic carboxylic acids, any substituent at the ortho position generally increases acidity. In addition, $NO_2$ at ortho also has a strong electron-withdrawing effect.

Hence,

4. Overall order

Combining all these results:

5. Match with options

This corresponds to Option D.