What type of cell to stimulate b cells to produce antibodies?

The correct answer is helper T cells.
The helper T cells are one of the main cells of the adaptive immune system. Their role is to help the activity of other immune cells by releasing T cell cytokines and they are required for almost all adaptive immune responses. The helper T cells activate B cells to secrete antibodies, stimulate macrophages to destroy ingested pathogens, and help in activation of cytotoxic T cells to kill infected target cells.

The answer is 0.01 meter