C. Sadness
Explanation:
In this dialogue, Macbeth asks the Doctor in despair if he has any antidote for a mind laden with guilt and sorrow which is the cause of a person's pain not some physical ailment.
This understanding that some mental grief is cause of a physical decay which the doctor will be unable to help him for is evident to Macbeth. Hence the emotion expressed is of sadness and helplessness.
It is obvious that such antidote to cleanse one's mind of sorrows does not exist and Macbeth's insistence on asking for it shows his sadness.