Hi!
Let's go through each answer to see if it's correct.
A. Mr. X
Mr. X is an unnamed man. X shows that the name of the man is unknown.
B. A C-sharp on the piano
That is a note on the piano.
C. A B+ grade
That is your score in school.
D. apartment 12a
An apartment room.
The answer is A. Mr. X
Hope this helps! :)
-Peredhel