**Core Concept**
Beta HCG is a hormone crucial for maintaining pregnancy, particularly during the early stages. It plays a vital role in implantation, embryonic development, and maintaining the corpus luteum, ensuring continuous progesterone production.

**Why the Correct Answer is Right**
Beta HCG is primarily produced by the placenta, specifically by the syncytiotrophoblast cells. This hormone is essential for implantation, as it helps to establish a successful pregnancy by promoting the growth of the embryo and the development of the placenta. Additionally, beta HCG maintains the corpus luteum, preventing its regression and ensuring continuous progesterone production, which is critical for fetal development.

**Why Each Wrong Option is Incorrect**
**Option A:** The ovary primarily produces estrogen and progesterone, not beta HCG.
**Option B:** The pituitary gland secretes LH (luteinizing hormone) and FSH (follicle-stimulating hormone), which stimulate the ovary to produce estrogen and progesterone but not beta HCG.
**Option C:** While the corpus luteum does produce progesterone, it does not produce beta HCG.

**Clinical Pearl / High-Yield Fact**
Beta HCG levels are commonly used as a marker for pregnancy and can also be used to monitor for potential issues such as ectopic pregnancies or miscarriages.

**✓ Correct Answer: D. Placenta**