Answer:
She has used the representativeness heuristic in making this assumption.
Explanation:
The representativeness heuristic occurs when a general assumption is made about something by grouping similarities between objects and categorizing them in the same group. It can be a form of stereotyping.
In this case, Becky has seen her roommate's blue hair dye as well as her side of the room's decoration. Based on this information, she pieces it together and thus assumes she has piercings and listens to a specific type of music, thus stereotyping her roommate.