Takumi Has to Tell David That His Attitude and Mannerisms

Takumi has to tell David that his attitude and mannerisms have offended some customers.He also must inform David that he is on probation for six weeks and will be fired if his behavior does not improve.Takumi should deliver this message in a(n)

A) email.
B) phone call.
C) video call.
D) personal meeting.
E) memo.