Here's why:

* Phagocytosis is a type of endocytosis where the cell engulfs large particles, like bacteria, by forming a vesicle around them. This vesicle then fuses with lysosomes, which contain enzymes that break down the bacteria.

* Exocytosis is the opposite process, where the cell expels material outside.

* Pinocytosis is the process of taking in fluids and dissolved molecules.

* Diffusion is the movement of molecules from high concentration to low concentration, and it doesn't involve the cell actively engulfing anything.

Therefore, phagocytosis is the specific type of bulk transport that white blood cells use to engulf and destroy bacteria.