Which one is the most acidic compound?

Hint: The acidity of a species depends on its power to remove hydrogen ions.

Explanation:

Step 1: Acidic character dependence

If an electron-withdrawing group is attached to a benzene ring then the group will attracts the ring electrons which facilitate the release of hydrogen ions and the species become more acidic.

If an electron-donating group is attached to a benzene ring then the group will supply electrons to the ring which does not release hydrogen ions easily and the species become least acidic.

Step 2: Acidity comparison

This compound has three electron-withdrawing groups $N{O}_2$. So it will withdraw the electrons from the $O-H$ bond and the bond breaks easily. So, it is most acidic.

This compound has one electron-withdrawing group $N{O}_2$. So, it will also pose higher acidity.

This compound has an electron-donating group so, it is the least acidic.

This compound is moderately acidic due to the presence of the hydroxyl group.
The order of acidity of the compounds is $A>D>C>B$

Correct Option : $A$