Final answer:
In the mechanism of phagocytosis, the step where the pathogen is killed by enzymatic action as the phagosome fuses with a lysosome to form a phagolysosome is called Lysosome fusion.
Step-by-step explanation:
Phagocytosis is a process by which phagocytes engulf and destroy pathogens. One of the steps in phagocytosis is the fusion of the phagosome, which contains the pathogen, with a lysosome, forming a phagolysosome. In the phagolysosome, the pathogen is killed by enzymatic action from the lysosome's digestive enzymes. Therefore, the correct answer is d) Lysosome fusion.