Which marker is associated with activated T Cells?

The marker CD25 is associated with activated T cells, particularly those that have been stimulated and are undergoing proliferation. CD25 is the alpha subunit of the interleukin-2 (IL-2) receptor and its expression is upregulated during T cell activation. Activated T cells, which respond to immune challenges, increase the expression of CD25 to enhance their ability to respond to IL-2, a critical growth factor that promotes T cell proliferation and survival.

In contrast, other markers mentioned in the question serve different functions: CD20 is primarily a B cell marker involved in the regulation of B cell activation and is not typically associated with T cells. CD2 is a marker that is present on T cells but does not specifically indicate activation; rather, it is involved in cell adhesion and is expressed on all T cells. CD34 is a marker associated with hematopoietic stem cells and endothelial cells, and is not related to T cell activation. Therefore, the distinct association of CD25 with activated T cells sets it apart as the correct answer in this context.