Final answer:
In the context provided, 'suffusing' means a sensation spreading throughout the body, so the correct answer is C. Spreading.
Step-by-step explanation:
The word suffusing in the context given ('even through your sleep, you feel a vague discomfort suffusing your body') refers to a feeling that is spreading throughout your body. Therefore, the correct answer is C. Spreading. This term implies a gradual process where something is permeating or becoming distributed throughout something else. It is different from clearing, fading, or diminishing, as those suggest a reduction or disappearance, whereas suffusing suggests an increase or proliferation of a sensation or condition.