**Core Concept**
The envelope protein gp120 is a crucial component of the HIV-1 virus, playing a key role in the initial stages of viral entry into host cells. It is responsible for binding to the CD4 receptor on T cells, facilitating the subsequent fusion of the viral envelope with the host cell membrane.

**Why the Correct Answer is Right**
The correct answer relates to the mechanism of HIV-1 entry into host cells. gp120 binds to the CD4 receptor on T cells, which triggers a conformational change in gp120, exposing the binding site for CCR5 or CXCR4 co-receptors. This interaction leads to the fusion of the viral envelope with the host cell membrane, allowing the viral core to enter the host cell. The binding of gp120 to CD4 is a critical step in the HIV-1 life cycle, and it is a major target for therapeutic interventions.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect, as gp120 does not directly interact with the host cell membrane. Its primary function is to bind to the CD4 receptor and co-receptors.
**Option B:** Incorrect, as gp41 is the HIV-1 protein responsible for the fusion of the viral envelope with the host cell membrane, not gp120.
**Option C:** Incorrect, as gp120 is not directly involved in the release of viral particles from the host cell. This process is mediated by the viral envelope and other host cell components.

**Clinical Pearl / High-Yield Fact**
gp120 is a prime target for therapeutic interventions, including vaccines and drugs that aim to block its interaction with the CD4 receptor or co-receptors. Understanding the role of gp120 in HIV-1 entry is crucial for developing effective strategies to prevent and treat HIV infection.

**Correct Answer:** C. It is involved in fusion of viral envelope with the host cell membrane.