Which of the following is the strongest acid?
A) butanoic acid
B) 2-methyl-propanoic acid
C) 2-fluoro-butanoic acid
D) 4-chloro-butanoic acid
2-fluorobutanoic acid is the strongest due to the strong -I effect of a very electronegative fluorine atom. Moreover, fluorine is in the 2nd position (close to the carboxylic acid group), which increases the acidity.