When molecules move down their concentration gradient, they move from where they are to where they are . Diffusion across a biological membrane is called .

they move from where they are more to where they are less.

Diffusion across a biological membrane is called passive diffusion.

Explanation:

When any molecules move across the cell membrane down their concentration gradient so they move from the area where their concentration is more to the area where their concentration is less and this process is called diffusion.

Diffusion across a cell membrane or biological membrane is called passive transport because no energy is needed to transport the molecules across the membrane as diffusion occurs along the concentration gradient.

the answer is c) an oak tree