What does the term "diffusion" refer to in the context of stochastic processes in finance?

$($A$)$ The tendency of assets to revert to their mean value.

$($B$)$ The expected difference between the value of the asset and the value of the derivative asset.

$($C$)$ The process of option pricing using the Black$-$Scholes model.

$($D$)$ The rate of change of the underlying asset price.

$($E$)$ The random fluctuations in asset prices over time.