What is C in the above picture?

The correct answer is Option (1) → Heavy chain

An antibody has two light chains and two heavy chains. The two heavy chains are the longer ones. They run from the tips of the two arms right down into the lower part of the Y, which is made only of heavy chains. The shorter light chains lie only along the outer side of the two arms.

Option (1) Heavy chain — Correct. C points to the long lower part of the Y. Only the two heavy chains run down into this part, so C is a heavy chain.

Option (2) Light chain — Incorrect. The light chains are the short chains on the outer side of the arms. They do not reach the lower part of the Y, so the label C cannot be a light chain. In the picture, B is the light chain.

Option (3) Antigen binding site — Incorrect. The antigen binding sites are at the tips of the two arms, where the antigen fits. In the picture, A points to this site.

NCERT Extract:
Acquired Immunity – “… Each antibody molecule has four peptide chains, two small called light chains and two longer called heavy chains. Hence, an antibody is represented as H₂L₂. Different types of antibodies are produced in our body. IgA, IgM, IgE, IgG are some of them. A cartoon of an antibody is given in Figure 7.4.”

Figure 7.4 Structure of an antibody molecule