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.
Question 1: Uds. (empezar)
Answer 1: Ustedes empiezan.
Question 2: Ud. (mirar)
Answer 2: Usted mira.
Question 3: Ud. (escribir)
Answer 3: Usted escribe.
Question 4: Ud. (comer)
Answer 4: Usted come.
Question 5: Ud. (beber)
Answer 5: Usted bebe.
Question 6: Ud. (respirar)
Answer 6: Usted respira.
Question 7: Usted (estudiar)
Answer 7: Usted estudia.
Question 8: Usted (escuchar)
Answer 8: Usted escucha.
Question 9: Ustedes (beber)
Answer 9: Ustedes beben.
Question 10: Usted (ver)
Answer 10: Usted ve.
Question 11: Uds. (usar)
Answer 11: Ustedes usan.
Question 11: Uds. (estudiar)
Answer 12: Ustedes estudian.
Question 13: Uds. (escribir)
Answer 13: Ustedes escriben.
Question 14: Uds. (comer)
Answer 14: Ustedes comen.
Question 15: Uds. (bailar)
Answer 15: Ustedes bailan.
1 c
2 a
3 b
4 d
5 d
6 b
7a
8b
9 c
10 c
11 d
12 d
13b
14 a
15 a
16 d
17 b
18 b
19 d
20 b