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An immunity, which takes place naturally due to physiology or genetic factors is termed innate immunity. It is not induced by vaccination or infection, however, functions to minimize the workload for the adaptive immune response.

High pH is not a barrier against the pathogens provided by the skin, it is the acidity of the skin, which acts as an obstruction against the pathogens.

The eyes and the mucus membranes, which are not protected by skin exhibit different methods of defense like mucus secretions and tears, which rinse away and trap pathogens, and the cilia in the respiratory tract and the nasal passages push the mucus with the pathogens out of the body.

Thus, options a, b and c are incorrect.

Before the stimulation of the immune factors, the skin works as a spontaneous, impassable barrier to harmful infectious pathogens.

The pathogens are activated or killed on the skin by drying out or desiccation and the acidity of the skin.

Hence, the correct answer is option d.