Read this sentence:
Kara knew that the giant black bird hovering in front of her
wasn't real and was only an illusion brought on as her
weakened mind battled to fight the fever.
Based on the context, what is the best definition of the word illusion?
A. Forces that cannot be explained by science or nature
B. A series of images occurring involuntarily in the mind during sleep
C. To make or become fragile or feeble
D. Something that appears to be real but is