Peach blossoms in early spring, lotus flowers in midsummer, red leaves in late autumn and snow falling in winter.

A, the catkin flying in early spring is a material movement, which belongs to mechanical movement. So a doesn't fit the question.

B. The fragrance of lotus in midsummer is the result of the movement of aromatic oil molecules.

C. Falling leaves are physical movements and belong to mechanical movements. So c doesn't fit the question.

D, the fog is diffuse in winter, and the water droplets liquefied by water vapor are moving, which belongs to mechanical movement.

So choose B.