Which is an example of diffusion

It depends on what your choices are but here are some:

Perfume is sprayed in one part of a room, yet soon it diffuses so that you can smell it everywhere.

A drop of food coloring diffuses throughout the water in a glass so that, eventually, the entire glass will be colored.

Explanation:

Example of diffusion is perfume sprayed in one corner of a room hence we can smell it everywhere in the room