Answer:
d. The apartment is right next to the mailboxes.
Step-by-step explanation:
As it is mentioned in the statement that every one knows where Ursula lives and also everyone knows her face. These things make it impossible for the first three statements to be true.
a. If her apartment is in the back corner, she will have interaction with people only while getting out and into the apartment which will not ensure that everyone will know her.
b. A door opening to a private hallway still doesn't ensure that she is this much social.
c. Upstair and far from stairwell also tells that only few people will know her, not all.
d. This one is true as she lives right next to mailboxes and every person in apartment tends to check their mailboxes and probability of knowing her face and where she lives is the highest.