Final answer:
The correct definition of cilium is 'hairlike motile projection from the surface of a cell,' which is option C. Cilia perform various functions, including the mucociliary escalator in the respiratory system, which moves mucus and trapped particles away from the lungs.
Step-by-step explanation:
Out of the options provided, the correct definition of the term cilium is C) hairlike motile projection from the surface of a cell. Cilia (plural for cilium) are short, hairlike projections that extend from the surface of some cells. These are structured with a core of microtubules arranged in a 9+2 array and are responsible for various functions depending on their location.
In the respiratory system, cilia are found lining the airways. Here, they function to catch and move particles and pathogens up and away from the lungs – a process called the mucociliary escalator. These ciliated epithelial cells secrete mucus that traps particles, which the cilia then help to move towards the throat to be expelled or swallowed.
Cilia also play important roles in other body systems. For instance, in the reproductive system, cilia located in the fallopian tubes help in moving the egg towards the uterus.