The correct answer is option 'C': the antigen binding site of an antibody is present at the variable region.

Antibodies, also known as immunoglobulins (Ig), are proteins produced by the immune system in response to the presence of foreign substances called antigens. Antibodies play a crucial role in the immune response by recognizing and binding specifically to antigens, thereby identifying them for destruction by other immune cells.

The structure of an antibody is composed of four polypeptide chains: two heavy chains and two light chains. Each chain consists of constant and variable regions. The constant region is responsible for the effector functions of the antibody, such as binding to immune cells and activating complement proteins. On the other hand, the variable region is responsible for antigen recognition and binding.

The variable region of an antibody is further divided into two parts: the variable heavy (VH) region and the variable light (VL) region. These regions contain hypervariable loops, also known as complementarity-determining regions (CDRs), which are responsible for antigen recognition and binding.

The antigen binding site, also referred to as the paratope, is formed by the CDRs of the variable regions. The CDRs are highly diverse and can form a wide range of shapes and surfaces, allowing antibodies to bind to antigens with high specificity and affinity. The antigen binding site is located at the tips of the Y-shaped antibody molecule, where the variable heavy and light chains come together.

The variable region of an antibody is encoded by multiple gene segments that undergo rearrangement during B cell development. This process generates a diverse repertoire of antibodies capable of recognizing a vast array of antigens.

In conclusion, the antigen binding site of an antibody is present at the variable region, specifically at the hypervariable loops or CDRs. This region is responsible for the specificity and affinity of antibody-antigen interactions, allowing the immune system to effectively recognize and eliminate foreign substances.