The phenomenon described in the question is known as "Social Loafing." This occurs when individuals exert less effort to achieve a goal when they work in a group compared to when they work alone. In the context of the question, when people clap in a large group, each participant tends to put in less effort due to the diffusion of responsibility among the group members, leading to a decrease in the noise output per person.
The other options:
Therefore, the correct answer is Social Loafing.