Which of the following is correct decreasing order of acidic strength for the compounds listed below?
I: p-chlorophenol, II: p-Cresol, III: p-nitrophenol, IV: p-methoxyphenol.

Acidic strength of phenols is directly proportional to the stability of the phenoxide ion formed after losing a proton.

Electron-withdrawing groups stabilize the phenoxide ion through $-I$ and $-R$ effects, thereby increasing the acidic strength. Electron-donating groups destabilize the phenoxide ion through $+I$, $+H$, and $+R$ effects, thereby decreasing the acidic strength.

In p-nitrophenol (III), the $-NO_2$ group exerts strong $-R$ and $-I$ effects, making it the most acidic.

In p-chlorophenol (I), the $-Cl$ group exerts a $-I$ effect which dominates over its $+R$ effect, making it more acidic than phenol but less acidic than p-nitrophenol.

In p-cresol (II), the $-CH_3$ group exerts $+I$ and hyperconjugation ($+H$) effects, decreasing the acidic strength compared to phenol.

In p-methoxyphenol (IV), the $-OCH_3$ group exerts a strong $+R$ effect which dominates over its $-I$ effect, making it the least acidic.

Thus, the decreasing order of acidic strength is III $>$ I $>$ II $>$ IV.

Answer: III $>$ I $>$ II $>$ IV