Failure to offer woman a seat on a crowded bus is usually considered a violation is

Breach of Folkways is the correct answer.

Folkway is a learned set of social behavior that is shared by people in society and considered essential to the welfare of community. It contains rules that regards the way of thinking, feeling, behaving and acting. Some good examples of folkways are offering a seat to women or the elderly on a crowded bus, shaking hands when being introduced to other people, holding the door for someone, and not interrupting people when they're busy or on the phone.