Final answer:
The primary function of the Fab region in an antibody is antigen binding, which is essential for the neutralization of pathogens and other immune functions. The correct option is A.
Step-by-step explanation:
The primary function of the Fab region in an antibody is A) Antigen binding. The Fab (fragment of antigen binding) region constitutes the 'arms' of the Y-shaped antibody molecule, each arm being capable of binding to specific epitopes on an antigen.
This binding is essential for various immune functions, such as neutralization of pathogens, opsonization, and antibody-dependent cell-mediated cytotoxicity. The variability of the amino acid sequences in the Fab region allows for the extensive diversity of antibodies, enabling the immune system to recognize a vast array of different antigens.