An Officer Wants to Enter a Certain Area of a Large

An officer wants to enter a certain area of a large apartment building to observe activities and seize any contraband he might happen across.A good rule of thumb for the officer is that

A) he may not enter if he has not been invited by a resident.
B) he may enter any area commonly used by visitors, businessmen, and delivery people to gain access and move about the building.
C) he may not enter without probable cause.
D) he may enter any area where incriminating evidence is immediately apparent.