Final answer:
The behavior of the motile bacterium that moves in a completely random manner is random movement.
Step-by-step explanation:
The behavior of the motile bacterium that moves in a completely random manner is random movement. Bacteria can move in response to various environmental signals such as light (phototaxis), magnetic fields (magnetotaxis), and chemical gradients (chemotaxis). However, when there is no chemical gradient, the movement of the bacterium becomes more random.