A transparent illuminated box contains small smoke particles and air - AQA - A-Level Physics - Question 11 - 2018 - Paper 2

The observation of Brownian motion is best explained by option B: Collisions between smoke particles and air molecules. This is because Brownian motion refers to the erratic movement of microscopic particles suspended in a fluid, caused by their collisions with the fast-moving molecules of the fluid. In this case, the smoke particles are colliding with the air molecules, resulting in their random motion.