¿Qué trae tu mamá a la fiesta? Now say your response using a direct object pronoun.
Translated:
Que trea tu mama a la fiesta ~ What does your mom bring to the party -
If "¿Qué trae tu mamá a la fiesta?" means "What does your mom bring to the party?", then you could respond with "Ella trae el pastel a la fiesta." , which means "She brings the cake to the party."