The best answer is D.
A carbon sink is anything that absorbs more carbon than it produces or releases.
The main carbon sinks found in nature are plants, the ocean and soil. Plants such as trees capture carbon dioxide from the atmosphere and use it in photosynthesis. Part of this carbon is transferred to soil as the trees and other plants die and decompose.
Due to the effectiveness of trees and other plants as carbon sinks, the Kyoto protocol proposed that emission reduction can be achieved through absorption of carbon dioxide by these plants.
