Question

for diffusion to occur, which condition must be met?
a concentration gradient must exist.
a hypotonic external environment must exist.
a hypertonic external environment must exist.
dynamic equilibrium must exist.

Explanation:

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. A concentration gradient (difference in concentration) is necessary for this movement. A hypotonic or hypertonic environment relates to osmosis (a special case of diffusion for water) but is not a general requirement for all diffusion. Dynamic equilibrium is a state after diffusion has occurred (when there is no net movement), not a condition for diffusion to start.

Answer:

A. A concentration gradient must exist.