Final answer:
Immunoglobulin isotype IgA is present in mucus secretions and is integral for protecting mucous membranes by trapping and neutralizing pathogens. The correct option is A.
Step-by-step explanation:
The immunoglobulin isotype that can be found in mucus secretions is IgA. This antibody plays a crucial role in the immune system by protecting mucous membranes.
Secretory IgA is most commonly found as a dimer, meaning two monomers joined by a secretory component, in various secretions including mucus, saliva, tears, and breast milk.
IgA antibodies are abundant in these fluids and are important for trapping pathogens and preventing infection of the mucosa by neutralizing them.