Final answer:
The major uncertainty in the results of global climate models is the net feedback of clouds. Changes in cloud cover and properties can have a significant impact on climate projections. Therefore, the correct option is c) Clouds.
Step-by-step explanation:
The major uncertainty in the results of global climate models is the net feedback of clouds. Clouds play a crucial role in climate by reflecting sunlight back into space, but they also trap heat, making their overall impact on global temperatures uncertain. Changes in cloud cover and properties can have a significant impact on climate projections.