Lymphocytes that activate B-cells and T-cells are

The correct option is D helper T-cells
The three major types of lymphocyte are T cells, B cells and natural killer (NK) cells. Lymphocytes can be identified by their large nucleus. T helper cells(T_H cells) assist other white blood cells in immunologic processes, including maturation of B cells into plasma cells and memory B cells and activation of cytotoxic T cells and macrophages These cells are also known as CD4⁺ T cells because they express the CD4 glycoprotein on their surfaces. Helper T cells become activated when they are presented with peptide antigens by MHC class two molecules, which are expressed on the surface of antigen- presenting cells (APCs).

So, the correct option is 'Option D'.