Where is the endogenous version of this hormone stored within the body?

A woman in labor continues to be dilated 2 cm after two hours in labor. She is given a synthetic version of a hormone to help dilate her cervix. Where is the endogenous version of this hormone stored within the body?

(A) Adrenal cortex
(B) Anterior pituitary gland
© Hypothalamus
(D) Mammary glands
(E) Posterior pituitary gland