**Core Concept**
Human chorionic gonadotrophin (hCG) is a hormone produced during pregnancy that is essential for maintaining the corpus luteum and, subsequently, the placenta. The measurement of hCG levels is crucial in various clinical scenarios, including pregnancy testing and monitoring.

**Why the Correct Answer is Right**
The most sensitive method of quantitative measurement of hCG is radioimmunoassay (RIA). RIA involves the use of radioactive markers to detect the presence of hCG antibodies, allowing for extremely sensitive detection of hCG levels. This method has a detection limit of as low as 0.1-0.2 mIU/mL, making it the most sensitive among various methods.

**Why Each Wrong Option is Incorrect**
* **Option A:** Immunoradiometric assay (IRMA) is another sensitive method, but it is not as sensitive as RIA. IRMA uses two antibodies to detect hCG, one attached to a solid phase and the other labeled with a radioactive marker.
* **Option B:** Enzyme-linked immunosorbent assay (ELISA) is a less sensitive method compared to RIA and IRMA. ELISA uses an enzyme-linked antibody to detect hCG, but it requires a higher concentration of hCG to produce a detectable signal.
* **Option C:** Chemiluminescent immunoassay (CLI) is a sensitive method, but it is not as widely used as RIA or IRMA. CLI uses chemiluminescent markers to detect hCG antibodies, but it requires specialized equipment.

**Clinical Pearl / High-Yield Fact**
When interpreting hCG levels, it's essential to remember that a significant increase in hCG levels is typically seen within 10-14 days after ovulation. A plateau or decrease in hCG levels may indicate a potential issue with the pregnancy.

**Correct Answer:** C. Chemiluminescent immunoassay (CLI)