An Officer Who Smells Marijuana as He Drives by a Car

An officer who smells marijuana as he drives by a car with an open window:

A) cannot compel the driver to stop the car so he can investigate without a warrant.
B) is limited to noting the license plate number of the vehicle and initiating a stake­out of the owner's home in order to gather additional evidence.
C) is authorized to stop the car to investigate without a warrant or probable cause because it is a plain search and outside the scope of the Fourth Amendment.
D) is authorized to stop and investigate because he has probable cause under the Fourth Amendment.