Ida b wells became an activist when she

Ida B. Wells became an activist when she was forced to change her seat on the train to where African Americans "belong".

Explanation:

Ida B. Wells bought a train ticket from Memphis to Nashville for first-class part of the train. But after the train control arrived she was asked to change her seat for the one in African American part of the train. At first she denied and rebelled greatly. She even hit one person. But at the end she needed to change the seat.

This event caused that she feels powerful sentiment against prejudice and begins to be activist and fight against it.