The antibodies found almost entirely and only on the surface of b cells (not secreted from them), and which always exist as monomers, are

These antibodies are IgD.

It is a monomeric antibody isotype that is expressed in the plasma membranes of immature B-lymphocytes. It is also produced in a secreted form that is found in small amounts in blood serum. IgD is co-expressed with IgM on the surface of the majority of mature B cells prior to antigenic stimulation and functions as a transmembrane antigen receptor.