What is crowd psychology

Crowd psychology is the broad study of how individual behavior is impacted when large crowds group together. This field of social science has progressed from the early examination of negative social groupings to the study of crowds in more socially proactive or emergency-type of environments.

answer; i do believe its (b) 12.5 years of age;

i believe the answer is:   psychological  experiment

in this experiment, wilhelm wundt became the first person that implement scientific method in analyzing the pyschological phenomenon.

due to this result, wilhelm wundt also able to make psychology to be acknowledged as a separate scientific field from biology.