What is a hapten?
a. a small molecule that has reactivity but lacks immunogenicity
b. the heavy chain of an immunoglobulin molecule
c. the antigen binding site of an immunoglobulin molecule
d. the part of a lyrmphoid organ where antigens are processed
e. a self antigen genetically encoded by the mhc molecule

Option (A).

Explanation:

Haptens may be defined as incomplete antigen that are not capable of inducing an immune response.

Hapten is a small molecule that reacts with specific antibody but only generates an immune response when these are conjugated with the carriers or proteins. Penicillin is a hapten and generates an immune response when conjugated with the carrier molecule.

Thus, the correct answer is option (A).