Final answer:
CD28 is the cell surface molecule that binds to CD80/86 and conveys a stimulatory signal during T-cell activation.
Step-by-step explanation:
The cell surface molecule that binds to CD80/86 and conveys a stimulatory signal during T-cell activation is CD28. CD28 is a protein receptor found on the surface of T cells and interacts with CD80/86 molecules on antigen-presenting cells (APCs). When CD28 on T cells binds to CD80/86 on APCs, it delivers a co-stimulatory signal that enhances T-cell activation and proliferation.