Mandy grew up believing that everyone from California was "weird." But at school, Mandy's new roommate and best friend is from California, so she reasons that maybe Californians aren't all that bad. What best accounts for her attitude shift?

Acclimatization accounts for her shift from her attitude.

Explanation:

Acclimatization often is responsible for taking away the cultural biases of people.

People are often thinking in terms of their own incredible biases with some places that are not always held up or are even logical.

To get these biases completely out of the way of the person they often need to talk to the people they have this bias about and understand that there is no key difference to them as such.

This is what happens in this case and she begins to lose her bias.