**Core Concept**
Decidual reaction and Arias-Stella reaction are two distinct histopathological changes observed in the endometrium during pregnancy, particularly in the context of a threatened or ectopic pregnancy. The decidual reaction is a response of the endometrium to progesterone, leading to thickening and vascularization, while the Arias-Stella reaction involves nuclear changes in the endometrial glandular cells.

**Why the Correct Answer is Right**
The hormone responsible for these reactions is human chorionic gonadotropin (hCG). hCG is produced by the placental syncytiotrophoblast and stimulates the corpus luteum to produce progesterone, which in turn causes the decidual reaction. The Arias-Stella reaction is also thought to be triggered by hCG, which may stimulate the endometrial glandular cells to proliferate and undergo nuclear changes. This is a critical mechanism for establishing and maintaining pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Not relevant to decidual reaction or Arias-Stella reaction.
**Option B:** Not directly related to the endometrial changes observed in pregnancy.
**Option C:** Not the primary hormone responsible for decidual reaction and Arias-Stella reaction.

**Clinical Pearl / High-Yield Fact**
The Arias-Stella reaction can be a useful diagnostic clue in cases of ectopic pregnancy, as it may be observed in endometrial biopsies or curettings. However, it is essential to consider the clinical context and other histopathological features to confirm the diagnosis.

**Correct Answer:** C. Human chorionic gonadotropin (hCG).