Final answer:
The false consensus effect occurs when individuals in a group believe that their own opinions and attitudes are more widely shared than they actually are.
Step-by-step explanation:
The correct answer is c) The false consensus effect. The false consensus effect occurs when individuals in a group believe that their own opinions and attitudes are more widely shared than they actually are. In the given scenario, the members of Jim Jones' group assumed that they knew what others were thinking, but their assumptions were wrong. They mistakenly believed that their own thoughts and attitudes were shared by the rest of the group, leading to the false consensus effect.