The correct answer is A. Nervous system
Step-by-step explanation:
Processes such as thinking, generating ideas, talking, or concentrating are related to the brain. Indeed, this organ is the one responsible for all of these processes and for controlling the function of other organs. Moreover, this important organ is part of the nervous system that processes stimuli and information. In this context, Chelsea's lack of concentration and other issues are related to her brain, and therefore her nervous system is the one affected.