Final answer:
CD4+ T cells are activated by the interaction between their T-cell receptors (TCRs) and MHC II molecules on antigen-presenting cells, with cytokines also playing a role in this process.
Step-by-step explanation:
Activation of CD4+ T Cells
CD4+ T cells, commonly known as helper T cells, are primarily activated by the interaction between their T-cell receptors (TCRs) and antigen-presenting cells (APCs) displaying peptides on major histocompatibility complex class II (MHC II) molecules. Upon encountering an APC, the CD4+ T cell's TCR binds to the MHC II molecule that is presenting an antigenic peptide. This binding is necessary for the activation of the naive helper T cell. In addition to this interaction, the activation process involves both the APC and the T cell releasing cytokines, which are signaling molecules that aid in the proliferation and differentiation of T cells.
The direct answer to the student's question is that CD4+ T cells are activated by Option 3) interaction between TCRs and MHC II. Option 4) interaction between CD4+ and MHC II is partially correct, as the interaction of CD4+ with MHC II is part of the larger binding event involving the TCR.