(A) When an air particle bombards a smoke particles, the smoke particle moves to the same direction as the air particle that hit it. When another air particle hits the smoke particle, it changes its direction to that of the second air particle, and so on. This is called Brownian motion.
(B) If the temperature of the surroundings is increased the air particles would gain kinetic energy resulting in more frequent collisions with the smoke particles. Therefore, the smoke particles would be seen moving haphazardly.
(C) The microscope helps magnify the smoke particles for a much more better observation. The lens help in focusing. As the air particles are really small and cannot be observed; therefore, smoke particles are used as they are easy to see and observe; thus, ensuring a better experiment.