IMO the answer should be C.

It's not that scientists had to remove the hormone from the pituitary glands. It's just that before the scientists had learned how to create it artificially, the source of such hormone was the pituitary gland.
Scientists' job was to synthesize the hormone.

In option D, "make a synthetic" is wordy as compared to "synthesize".

What is the OA?

imo C

i would go for option C

A: removing painstakingly is not clear- who removed what
B: not clear if the scientists removed it themselves
C: correctly identifies before scientists learned about something, what was the practise
D: not clear what"it" refers to
E: not clear what"it" refers to