Final answer:
The thymocyte will become a CD8 T cell (option a) during positive selection if it selects an MHC class I molecule. This determines the cell's destiny, leading it to become a cytotoxic T cell that expresses CD8 and binds to MHC class I on target cells.
Step-by-step explanation:
During positive selection, if the MHC molecule selected is class I, then the thymocyte will become a CD8 T cell. This process takes place in the thymus where thymocytes that have both CD4 and CD8 receptors, known as double positives, interact with thymic epithelial cells presenting MHC proteins. If a thymocyte successfully binds to a class I MHC molecule, it will be selected for and will receive signal to cease expression of the CD4 receptor and continue expressing CD8.
Consequently, the cell becomes a CD8+ cytotoxic T cell capable of binding to MHC class I on target cells. Those that bind to MHC class II will enhance expression of CD4 and become CD4+ helper T cells. Essentially, the fate of the thymocyte, whether it becomes a helper T cell or a cytotoxic T cell, is determined by its ability to bind either class I or class II MHC molecules, respectively.