Final answer:
The incorrect statement is that an antibody's constant region is the antigen-binding part; it is the variable region that binds to the antigen.
Step-by-step explanation:
The question asks which statement about antibodies and antigens is NOT correct. The correct statements are (a) Antibodies are secreted by plasma cells, (b) An antibody binds to a small part of an antigen known as an epitope, and (c) An antibody is an immunoglobulin protein. However, statement (d) is incorrect because an antibody's variable region, not the constant region, is the antigen-binding part. The constant region of the antibody molecule serves other functions, such as the recruitment of other cells of the immune system to help eliminate the antigen.