The correct answer is group polarization.
Group polarization refers to the phenomenon where individuals in a group setting have ideas that become more extreme and radical compared to their actual and personally held judgments and beliefs. In other words, being part of a group influences a member's beliefs. In this instance, Tabitha personally had doubts and lack of clarity regarding the new health care law signed by president Obama, however, after hearing the opinion of her four roommates on this matter, her opinion was influenced by them and matched their option, thus, demonstrating the effect of group polarization.