Step 1: Understanding the pregnancy hormone.
Progesterone is known as the pregnancy hormone as it plays a crucial role in maintaining pregnancy by supporting the uterine lining and preventing contractions.
Step 2: Analyzing the options.
(A) FSH: Follicle-stimulating hormone is involved in the regulation of the menstrual cycle, but not directly in pregnancy.
(B) ACTH: Adrenocorticotropic hormone stimulates the adrenal glands but is not the primary hormone related to pregnancy.
(C) Estrogen: While important during pregnancy, estrogen is not the primary hormone responsible for sustaining pregnancy.
(D) Progesterone: Correct. Progesterone is critical for maintaining pregnancy.
Step 3: Conclusion.
The correct answer is (D) Progesterone.