**Core Concept**
The question refers to the **gp120 antigen** on the HIV surface, which plays a crucial role in the virus's attachment and entry into host cells. This antigen is part of the **envelope protein** of HIV, facilitating the initial steps of infection. Understanding the function of **gp120** is essential in comprehending the HIV infection process.

**Why the Correct Answer is Right**
The correct answer is related to the function of **gp120** in binding to **CD4 receptors** on the surface of T-helper cells, which is a critical step in the HIV entry process. This binding induces a conformational change in the **gp120**, allowing it to bind to **chemokine co-receptors** such as **CCR5** or **CXCR4**, which facilitates the fusion of the viral envelope with the host cell membrane.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not accurately describe the primary function of **gp120**.
**Option B:** Similarly, this option is incorrect as it misrepresents the role of **gp120** in the HIV infection process.
**Option D:** This option is also incorrect as it fails to accurately describe the consequence of **gp120** antigen presence on the HIV surface.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **gp120**'s interaction with **CD4** and **chemokine co-receptors** is a target for therapeutic intervention, with several drugs aiming to block these interactions to prevent HIV entry into host cells.

**Correct Answer:** D. Binding to CD4 receptors