Final answer:
The mucous membrane that lines the underside of each eyelid and covers the eyeball is known as the conjunctiva, which helps to lubricate and protect the eye. The correct option is a.
Step-by-step explanation:
The mucous membrane that lines the underside of each eyelid and covers the surface of the eyeball is called the conjunctiva. This membrane is vital as it serves to protect the eye from pathogens and debris, and maintains the moisture of the eye along with the tear film. The conjunctiva contains mucous-secreting cells, which produce the mucus that helps to keep the eye lubricated.
Infections such as conjunctivitis, commonly known as pinkeye, can occur when this membrane becomes inflamed. Furthermore, the conjunctiva is a part of the eye's defense system, where it may harbor a normal microbiota that helps in protecting against the onset of pathogenic infections. Hence, a is the correct option.