Answer: Bandwagon
Bandwagon fallacy assumes that the opinion of the majority is always right. It is also called an appeal to popularity, or argumentum ad populum. The problem with this line of thinking is that this type of argument only proves that a position is "popular," but it says nothing about whether it is "true." In this case, the argument offered for eating Lunch Nibbles is simply that everyone is doing it, instead of discussing the actual benefits of Lunch Nibbles.