Beatrice turns sixteen and has to make the choice of what faction she will be in for the rest of her life. She is determined to stay in Abegnation when she reaches the time to chose.
But she doesn't exactly chose what she thought she would. Her test results for the recommendation of what faction she should chose stated that she wasn't any of the factions.
She was Divergent.
Her thoughts quickly veered toward Dauntless; the brave.
When she becomes Dauntless they give her the option to change her name and she happily takes it; changing her name to Tris