Final answer:
The cell surface molecule that stabilizes interactions of TCR and MHC during T-cell activation is CD4.
Step-by-step explanation:
The cell surface molecule that stabilizes interactions of TCR (T-cell receptor) and MHC (major histocompatibility complex) during T-cell activation is CD4 (option A).
CD4 is a co-receptor protein found on the surface of T-helper cells. It binds to the MHC class II molecule on antigen-presenting cells (APCs), like macrophages or dendritic cells, along with the TCR binding to antigen. The interaction between CD4 and MHC stabilizes the immune synapse, facilitating efficient T-cell activation and the initiation of an immune response.
In contrast, CD28 (option B) is involved in T-cell co-stimulation, CD45 (option C) is a phosphatase involved in T-cell activation, and CD152 (CTLA-4) (option D) is a co-inhibitory molecule that downregulates T-cell activation.