Which of the Following Would Best Describe a Norm

Which of the following would best describe a norm?

A) Norms are the laws of a culture. They are behaviours that are so important that they are written into law.
B) Norms are so "normal" to a group that even small children can describe them in detail to outsiders.
C) Norms are the universal behaviours that set us apart as humans irrespective of the culture a person happens to live in. Norms allow persons to understand at least parts of other cultures.
D) A norm is what is considered normal within a group. Norms are so taken for granted that most people don't know what their norms are until they see one violated.