What is the watery secretion produced by mucous membranes that lubricates body openings?

Mucus is the correct answer because it is a viscous, slippery substance produced by mucous membranes that serve various functions in the body. Primarily, mucus acts as a lubricant, helping to keep body openings moist and facilitating the movement of substances. This is particularly important in areas such as the respiratory and digestive tracts, where mucus helps trap pathogens and debris, protects tissues, and aids in the smooth passage of food and air.

In addition to its lubricating properties, mucus also plays a role in protecting underlying tissues from damage and infection, which is essential for maintaining overall health.

To clarify the other options: Plasma refers to the liquid component of blood, containing water, electrolytes, proteins, and nutrients, but it does not specifically relate to the lubrication of body openings. Serum is another component of blood that remains after clotting, and it also does not provide the lubricating function associated with mucous membranes. Synovial fluid, while important for lubricating joints, is produced specifically in synovial joints and does not serve the general lubrication function throughout the body openings like mucus does.