Among the following compounds, strongest acid is

Due to the presene of acidic -OH Group.The acidity of a compound depends upon its ability to donate a proton. When a proton or hydrogen is donated by a compound it will form an anion and the stability of an anion is directly proportional to the acidic strength of a compound.
The electron-donating groups destabilize the anion formed. This happens exactly the opposite in the case of the -I effect as it increases the acidity of the compound. Hence the plus I groups must have least influence or the minus I must have highest influence.

Alcohols are less acidic than phenols but more that alkynes.