The specific region on an antigen that an antibody recognizes and binds to is called the epitope, or antigenic determinant. An epitope is usually 5-8 amino acids long on the surface of the protein. Proteins are three dimensionally folded structures, and an epitope may only be recognized in its form as it exists in solution, or its native form. The antigen binding portion of an antibody varies extensively among secreted antibodies, and this length of sequence is known as the variable region. The variable region construction during antibody production in the B cell is what enables antibodies to be generated against an infinite variety of antigens. The other biological properties of the antibody and its role in signaling to other immune cells are determined by the constant regions of the heavy chains.So, the correct option is 'Variable regions'.