Answer:
It is caused by air molecules colliding with smoke particles
Step-by-step explanation:
To start with, Brownian motion is the erratic or irregular movement of very small particles in a medium due to continuous bombardment by molecules that surround the medium.
Thus, smoke moves in a Zig-Zag manner (Brownian motion) due to continious air bombardment. If it were pollen grain suspended in water, the erratic movement (Brownian motion) of the grains is caused by bombardment from water molecules.