Let us analyze each term separately. Law is a binding rule that regulates some aspect of life. There are collective social behaviors like the celebrations of 4th of July that are clearly not law (not binding, no fine or jail if you do not attend). Custom is the correct answer and the sentence given is a textbook definition of it. Conformity means the tendency to follow the lead of others and do things the way you see others do them, the tendency to abstain from innovating. While conformity plays a role in establishing customs, it does not fit our given description. Finally, deviance is the status where one deviates from the common standard and does something completely different. It is rather uncommon and this divergence can be treated with mixed feelings (sociologically too), but this term is again somewhat irrelevant to the question.
