Answer:
Their family doctor
Step-by-step explanation:
In the context, although Cassy is eating and taking food on time and regularly, She often becomes lethargic and falls sleep in the class. Resting for while is also not helping her to her improve her condition.
Therefore, I would recommend Cassy's parents to take her to their family doctor so that the doctor can investigate an diagnose her problem. Their family doctor will have Cassy's previous reports and medical treatment if any and will be aware of the reason for this. The family doctor knows Cassy for a while and her medical history, so it will be easy for him to detect her problem.