The acid strength of saturated aliphatic carboxylic acids depends mainly upon the inductive effect of the substituent and its position w.r.t. the -COOH group. Where as electron donating substituents tend to increase the acid strength. The acidic strength of aromatic carboxylic acids, on the other hand, depends upon both the inductive and the resonance effect of the substituents.

Which is the strongest acid among the following? 

m-CH₃OC₆H₄COOH

Since in C₆H₅COOH, the -COOH group is attached to more electronegative ^sp2 carbon of the C6H₅ ring while in CH₃COOH, it is attached to less electronegative  sp³ carbon of CH₃ group, therefore, benzoic acid is stronger acid than acetic acid. Since methoxy group acts as an electron withdrawing group at meta position but electron donating group at para position, therefore, m-methoxy benzoic acid is even more acidic than benzoic acid.