Final answer:
Mucus is the substance composed of glandular secretions, salts, cells, and leukocytes that protects internal surfaces of the body by trapping microbes and debris. Option d.
Step-by-step explanation:
The slime-like substance described in the question is mucus. Mucus is a viscous secretion produced by goblet cells and glands within mucous membranes found throughout the body. This secretion is generally composed of glandular secretions, salts, cells, and leukocytes. Its main function is to trap and remove microbes and debris from the body, and it also plays a significant role in providing a moist layer to various internal surfaces of the body, thus ensuring their protection against pathogens and facilitating smoother functions.