What effect does an increase in group size have on diffusion of responsibility? a. increased group size reduces perceived individual responsibility. b. increased group size increases perceived individual responsibility. c. increased group size does not affect perceived individual responsibility. d. increased group size forces one to take individual responsibility.

Considering that it can't be B because a bigger group increases the sense of a collective, it can't be C because no change is highly unlikely if not impossible, and it can't be D because someone always takes the responsibility considering that any group has some sort of leadership or organization, we can conclude that it is A. Increased group size reduces perceived individual responsibility.

A good way to understand it better is reading about the bystander apathy which somewhat explains this behavior.