A student sitting in the back row opened a bottle of foul-smelling perfume and dabbed it on her wrists. One by one (beginning from the back of the room) the students began to cough due to the foul smell. This phenomena was due to:

Diffusion

Explanation:

Diffusion is the movement of a substance (majorly liquids and gases) from a region of high concentration to a region of low concentration. Liquid and gases excluding solids can undergo diffusion because of their ability to move randomly. Diffusion is driven by a gradient in concentration.

For example gases can easily diffuse quickly and easily into another gas, because of its ability to move randomly. The foul air from the back room (higher concentration) was able to quickly spread into the clean air at the front row (low concentration) by diffusion