**Core Concept**
The question is testing knowledge on the diagnostic tests available for detecting the Human Immunodeficiency Virus (HIV), specifically the AIDS virus. HIV is a retrovirus that causes acquired immunodeficiency syndrome (AIDS) by targeting the CD4+ T lymphocytes in the immune system.

**Why the Correct Answer is Right**
The correct answer is an enzyme-linked immunosorbent assay (ELISA), which is a widely used screening test for HIV detection. This test detects the presence of antibodies against HIV in the blood. ELISA works by using a capture antibody to bind to the HIV antigen, followed by a detection antibody conjugated to an enzyme that reacts with a substrate to produce a colorimetric signal. This signal is proportional to the concentration of HIV antibodies in the sample.

**Why Each Wrong Option is Incorrect**
* **Option A:** Western Blot is a confirmatory test for HIV, not a screening test. It is used to confirm the presence of HIV antibodies in samples that have tested positive on ELISA.
* **Option B:** Polymerase Chain Reaction (PCR) is a molecular diagnostic test that detects the genetic material of HIV, but it is not a primary screening test for HIV detection.
* **Option C:** Rapid Tests are point-of-care tests that use a lateral flow assay to detect HIV antibodies, but they are not as sensitive as ELISA and are often used as a screening tool in low-resource settings.

**Clinical Pearl / High-Yield Fact**
It is essential to note that a positive result on an HIV screening test should be confirmed with a second test, such as Western Blot, to ensure accuracy.

**Correct Answer: A. ELISA. Enzyme-linked immunosorbent assay.**