Correct Option: D

Antibodies are secreted by a type of Lymphocytes (White Blood cell). Antibodies can occur in two physical forms, a soluble form that is secreted from the cell, and a membrane-bound form that is attached to the surface of a B cell and is referred to as the B cell receptor (BCR). An antibody (Ab), also known as an immunoglobulin (Ig), is a large Y-shaped protein produced by lymphocytes that is used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, called an antigen.