Answer:
A. Su
Explanation:
Because Su literally translates to their. Tu means you/your and mi means mine/my.
If it were casas, then you just modify the pronoun to sus.
The answer would be su.
Vamos a su casa.
Tu makes the most sense, and it is referring to the other person, in the conversation. The english translation of "Vamos a su casa." is "Let's go to your house." So su is the best answer
Hope this helps
ヽ(*・ω・)ﾉ
