The correct answer is B) green. Under the IALA-B Buoyage System, when entering from seaward, a buoy that should be left to port will be green in color. This is because the IALA-B system follows the principle of "red right returning," meaning that when returning to port or entering from seaward, red buoys should be kept to the starboard (right) side. Consequently, green buoys will be kept to the port (left) side when entering from the sea. The other answer choices are incorrect because red buoys are kept to the starboard side, black buoys are not part of the IALA-B system, and yellow buoys have a different meaning in the IALA-B system.