Which word from the paragraph best helps the reader understand the meaning of the underlined word?

Natalie mixed blue food coloring into the vanilla frosting. The diffusion of color was immediate. At first it was a light blue. But as she added more coloring, the spread of blue became darker in the frosting. It looked great on the giant cookie.

A.
frosting

B.
spread

C.
cookie

D.
coloring

the underlined word is diffusion.