Right answer: Usted
Step-by-step explanation:
The 2nd person in singular you has two forms of speaking in Spanish (Castilian):
Tú
It is used with friends, with relatives and in informal situations
Usted
It is used with people in formal situations: strangers, with elderly people, with people with a degree, such as Dr. or Dra., teacher, Miss, Mr, Mrs.
Therefore, the answer is A.
The other options are incorrect because:
él (he) is the 3rd person in singular
tú (you) is the informal of the 2nd person in singular (as explained above)
ustedes (you) is the 2nd person in plural