Answer: bout to lose brain cells on this
Explanation:
Pituitary gland I do believe
Being underweight can contribute to a very frail and weakened immune system. It can make one feel tired mentally. And can also affect the bones by making them very brittle and frail sometimes.
Pretty confident that it is A, judging by that the word composition is used as a combination or a ratio of 2 things. Also none of the other options make sense.