Answer
(a)
Work Step by Step
A negative feedback is a self-regulating mechanism that counteracts an initial change. In the context of global warming, increased absorption of atmospheric $CO_2$ by the ocean is a negative feedback because it helps to reduce the concentration of $CO_2$ in the atmosphere. The ocean acts as a carbon sink, absorbing a significant amount of $CO_2$ from the atmosphere, which helps to mitigate the greenhouse effect and slow down the rate of global warming.
The answer is (a).
