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. What is the most legitimate "good rule of thumb" for the officer?

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