Final answer:
The major structural protein of HIV-1 encoded by the gag gene is p24, which is part of the virus's capsid, while gp41 and gp120 are glycoproteins involved in viral attachment to host cells.
Step-by-step explanation:
The major structural protein (core) of the human immunodeficiency virus type 1 (HIV-1) encoded by the gag gene is p24. This protein is a part of the virus's inner structure, forming the viral core or capsid. The other options listed, gp41 and gp120, are indeed associated with HIV-1, but they are glycoproteins embedded in the virus's envelope, not the core structural protein encoded by the gag gene. The gp41 and gp120 proteins are crucial for the virus's ability to attach to and enter host cells. Specifically, gp120 binds to the CD4 receptor, a glycoprotein on the surfaces of T cells, which facilitates the initial interaction required for HIV to infect the cell.