**Core Concept**
The decidual reaction and Arias-Stella reaction are histopathological changes observed in the endometrium, particularly in the context of early pregnancy. These changes are triggered by hormonal influences that promote endometrial growth and differentiation.

**Why the Correct Answer is Right**
The correct answer is **Progesterone** because it plays a crucial role in preparing the endometrium for implantation of the fertilized ovum. Progesterone induces the decidual reaction, characterized by the transformation of endometrial glandular epithelial cells into decidual cells. This process involves the deposition of glycogen, the appearance of secretory vacuoles, and the development of a distinct cell morphology. The Arias-Stella reaction, which is a specific type of endometrial change, is also influenced by progesterone, leading to the appearance of characteristic nuclear pseudoinclusions and hyperchromasia in glandular epithelial cells. The progesterone-induced changes in the endometrium create an optimal environment for implantation and early embryonic development.

**Why Each Wrong Option is Incorrect**
**Option A:** Oestrogen is involved in the proliferation of the endometrium but does not induce the decidual reaction. Oestrogen and progesterone work in synergy to prepare the endometrium for implantation, but progesterone is the primary hormone responsible for the decidual reaction.

**Option C:** Human Chorionic Gonadotropin (hCG) is produced by the placenta during pregnancy and supports the corpus luteum, maintaining progesterone production. However, hCG is not directly responsible for the decidual reaction or the Arias-Stella reaction.

**Option D:** Human Placental Lactogen (hPL) is a hormone produced by the placenta that plays a role in fetal growth and development. However, hPL is not directly involved in the decidual reaction or the Arias-Stella reaction.

**Clinical Pearl / High-Yield Fact**
In cases of ectopic pregnancy, the Arias-Stella reaction can be observed in the endometrium, which can lead to diagnostic confusion with intrauterine pregnancy. However, the presence of trophoblastic tissue or a visible ectopic pregnancy on imaging studies can help distinguish between these conditions.

**✓ Correct Answer: B. Progesterone**