The new morals and manners of the 1920s were reflected in a number of different things. However, since you tagged this with “women’s rights,” I will assume that the answer that you are looking for is the “flappers.” These young women were one of the most noticeable reflections of the new ways of the 1920s.
During the 1920s, young women started to act in ways that were much less reserved and “proper” than had previously been the case. They started to wear shorter dresses. They started to wear more cosmetics. They started to smoke and drink in public. They danced new dances (to new jazz music) that were seen as improper and suggestive. They were much less bound by traditional ideas of sexual morality.
Because they acted in all of these untraditional ways, the “flappers” became a symbol of the changes of the 1920s. Therefore, the flappers are the most likely answer to this question.