Which of the following is a common source of biological contamination?

Biological contamination primarily refers to the presence of harmful microorganisms or biological agents in food that can lead to foodborne illnesses. Parasites are a significant source of such contamination, as they are living organisms that can invade the body of a host and cause diseases. Common examples of foodborne parasites include Giardia, Trichinella, and various types of worms.

While other options are related to contamination in the food environment, they represent different categories. Chemicals pertain to chemical contamination, typically from pesticides or cleaning agents. Broken glass is a physical contaminant, which may cause injury but does not introduce microorganisms. Pest droppings, on the other hand, can introduce both physical and biological contaminants, but parasites specifically refer to the living organisms that cause illness in humans. Thus, in the context of biological contamination, parasites are indeed a key source and the correct choice.