Final answer:
The term 'agnosia' contains the word part 'Gnos' which signifies knowledge, while the prefix 'a-' implies a lack thereof, referencing a condition of not knowing or recognizing.
Step-by-step explanation:
In the term "agnosia," the word part that means knowledge is b) Gnos. This comes from the Greek word 'gnosis' meaning knowledge or knowing. In contrast, the prefix 'a-' typically denotes a lack or absence of something. Therefore, 'agnosia' actually refers to the absence of knowledge, specifically relating to a neurological disorder characterized by the inability to recognize and interpret sensory stimuli. In the context of brain disorders like prosopagnosia, a person suffers from a specific type of agnosia where they cannot recognize faces, which can greatly impact their social interactions and ability to recognize even the most familiar individuals, such as family members or celebrities.