Inflammation
a is caused by bacterial activity to enhance the spread of disease
b brings more leukocytes to the sight of infection
c is caused by viral activity to enhance the spread of the disease
d slows the healing process with swelling that can impair bodily function

brings more leukocytes to the site of infection

Explanation:

Inflammation is a nonspecific immune response of the body to tissue damage or entry of pathogens in the body, etc. Inflammation is an immune response to dispose of microbes, toxins, or foreign material at the site of injury or infection and thereby, to prevent their spread to other tissues. Mast cells release vasodilation substances such as histamine which in turn dilates blood vessels. Histamine also increases the permeability of blood vessels to allow the leakage of tissue fluid leaks from the vessel.

The buildup of the tissue fluid causes swelling in the area which in turn stimulates pain receptors. Neutrophils and monocytes (two different types of leukocytes) are allowed to squeeze through the permeable capillary wall. These white blood cells kill the pathogens and destroy the toxins by the process of phagocytosis. Monocytes transform into macrophages which in turn phagocytize pathogens and stimulate the immune response.