Question

Which of the 3 substances ( benzoic acid, benzanilide, and acetanilide) is the most polar and least polar

Answer 1

more hydrogens and carbons, less polar
the more electronegative the substituents are, the more polar

p.s. benzoic acid and benzanilide look very similar, so compare the polarity of their side groups

Answer 2

It might help to draw out the resonance structures. The thing to note here is that resonance reduces polarity by spreading charges out; molecules with more resonance (hence more resonance structures), tend to be more stable and less reactive.