Answer:
diffusion of responsibility.
Step-by-step explanation:
Diffusion of responsibility is a phenomenon where a person is less likely to take responsibility for action when they are in the presence of a large group of people. This happens because the individual assumes that others are either responsible for taking action or have already done so.
Since they are part of a big group of people, people will tend to assume that someone else will probably help or, if nobody is helping, that the situation is not that serious. Thus, they don't feel pressured to respond.
In this example, Luke was robbed. He realized that many people had been watching the incident but no one tried to help him. In this case, people were watching but they probably assumed some other person would help him and they didn't feel pressured to respond. Therefore, this would be an example of diffusion of responsibility.