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The correct answer is mucous membranes. Mucous membranes play a crucial role in the body's nonspecific defense mechanisms against microbial invasion. They line various cavities and organs, such as the respiratory, gastrointestinal, and urogenital tracts, providing a barrier to pathogens. The mucus secreted by these membranes helps trap pathogens and microorganisms, preventing them from entering the body and facilitating their removal through ciliary action or other means.

While the other types of membranes mentioned also serve important functions, they do not directly contribute to the nonspecific defense mechanism in the same way as mucous membranes. For example, intestinal membranes are involved in nutrient absorption and digestion, respiratory membranes facilitate gas exchange, and nervous system components do not play a role in the physical barrier against microbes. Thus, mucous membranes are recognized as a key component in the first line of defense in the immune system.