Answer:
a. hippocampus
Step-by-step explanation:
Hippocampus: The term hippocampus is defined as a complex brain structure which is deeply embedded into an individual's temporal lobe of the brain.
The hippocampus plays an important role in memory, emotion, motivation, and learning. It is considered as a vulnerable and plastic structure that can be affected or can be damaged through various stimuli.
If the hippocampus of an individual gets damaged then he or she will experience a memory loss or will not be able to form new memories.
In the question above, Christopher's hippocampus was damaged.