For diffusion to occur, which condition must be met?

a) a concentration gradient must exist.
b) a hypotonic external environment is required.
c) a hypertonic external environment is required.
d) dynamic equilibrium must exist.

I think the correct answer from the choices listed above is option A. For diffusion to occur, the condition that must be met is that a concentration gradient must exist. This gradient is what drives the diffusion process. Without it, no diffusion can happen.