Answer:
Tris's mother knew so much about the Dauntless compound because she herself was born Dauntless to a Dauntless father.
Step-by-step explanation:
The debut novel of Veronica Roth, Divergent features a dystopian Chicago and the life of its protagonist Beatric Tris Prior.
Natalie Prior, at the end of the in chapter 15, asks Tris to bring a piece of cake for her, saying the chocolate is delicious. Natalie Prior is the mother of Tris Prior in the novel. The reason that Tris's mother know so much about the Dauntless compound, the ranking, and chocolate because she herself was born Dauntless to a Dauntless father.
Natalie is herself a divergent like her daughter, Tris. She chose the Abnegation faction on the advice of her mother.