Tu makes the most sense, and it is referring to the other person, in the conversation. The english translation of "Vamos a s<u>u</u> casa." is "Let's go to your house." So su is the best answer
No need to worry her are the answers For the I it’s yo For the we it’s ellos ellas For the he and she it’s el, Ella For the they you it’s nosotros, Nosotras
