Final answer:
The major player in Earth's climate system because of its capacity to absorb heat is the ocean. Therefore, the correct option is B. ocean.
Step-by-step explanation:
The major player in Earth's climate system because of its capacity to absorb heat is the ocean. The ocean acts as a thermal brake on fast heating and can absorb a tremendous amount of energy with very little resulting temperature change. The large heat capacity of water allows for the stabilization of temperatures on Earth and contributes to milder climates closer to large bodies of water, such as oceans.