Final answer:
Elaina's brain tumor is most likely affecting the hypothalamus, which is responsible for regulating various bodily functions.
Step-by-step explanation:
The symptoms experienced by Elaina, such as feeling very cold, almost fainting, a reduced appetite, and reduced sexual desire, suggest that her brain tumor is most likely affecting the hypothalamus. The hypothalamus is responsible for regulating various bodily functions, including temperature regulation, hunger and satiety, and sexual behavior. Disruption of the hypothalamus can lead to changes in these functions, resulting in the symptoms Elaina is experiencing.