1. Identify the acidic functional groups

   We compare the acidity of the given compounds:

   • A: $p$-nitrophenol
   • B: $p$-hydroxybenzoic acid
   • C: $o$-hydroxybenzoic acid
   • D: $p$-toluic acid

   Phenols are generally less acidic than carboxylic acids.
   Therefore, options B, C, D (carboxylic acids) are expected to be more acidic than A.

2. Eliminate the weaker acid

   • $p$-nitrophenol has acidity enhanced by the electron-withdrawing $-NO_2$ group.
   • But even substituted phenols usually have $pK_a \approx 7$ to $8$.
   • Carboxylic acids typically have $pK_a \approx 4$ to $5$.

   Hence, A is not the most acidic.

3. Compare the carboxylic acids: B, C, D

   • D: $p$-toluic acid has a $-CH_3$ group at para position.

     • $-CH_3$ is electron-donating by $+I$ effect.
     • It destabilizes the carboxylate ion slightly.
     • So acidity decreases.

   • B: $p$-hydroxybenzoic acid has an $-OH$ group at para position.

     • The $-OH$ group has a weak $-I$ effect but a strong $+R$ effect.
     • At para position, resonance donation dominates.
     • This donates electron density to the ring and reduces acidity of the carboxylic acid compared to a strongly withdrawing substituent.

   • C: $o$-hydroxybenzoic acid is salicylic acid.

     • It is known to be more acidic than benzoic acid and its para-hydroxy isomer.
     • The key reason is the ortho effect.
     • Also, the conjugate base is stabilized due to special interactions in the ortho position.
     • In salicylic acid, intramolecular hydrogen bonding and ortho substitution favor greater acidity.

4. Use known acidity order

   A standard order is: $o\text{-hydroxybenzoic acid} > benzoic\ acid > p\text{-hydroxybenzoic acid}$ and also $o\text{-hydroxybenzoic acid} > p\text{-toluic acid}$

   Thus, among all given compounds, the most acidic is: $\boxed{o\text{-hydroxybenzoic acid}}$

5. Option check

   • A: No
   • B: No
   • C: Yes
   • D: No

Therefore, the correct option is C.