Which of the following is a type of leukocyte?

Neutrophils are a type of leukocyte, which is a category of white blood cells that play a crucial role in the immune system. They are vital for the body's defense against infections, particularly bacterial infections, and are often the first responders to sites of inflammation or injury. Neutrophils account for a significant portion of the white blood cell count and are instrumental in phagocytosis, the process by which they engulf and digest pathogens.

The other alternatives do not fit into the category of leukocytes. Fibrin is a protein involved in clot formation during the coagulation process, thrombin is an enzyme that converts fibrinogen into fibrin during blood clotting, and cytokines are signaling molecules used by the immune system to communicate between cells, but they are not cells themselves. Understanding the role of neutrophils and other leukocytes is essential in recognizing how the body responds to infection and injury.