Final answer:
Farmland with irrigation is not an important carbon sink, unlike rainforests, heavily wooded forests, oceans, and grassy plains.
Step-by-step explanation:
Carbon sinks are natural or artificial reservoirs that absorb and store carbon dioxide from the atmosphere. The question asks which of the options is not an important carbon sink. The correct answer is b. farmland with irrigation. While rainforests, heavily wooded forests, oceans, and grassy plains are all important carbon sinks, farmland with irrigation is not a significant carbon sink.