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This is kind of a two part question:
1) the first part is given as "moving from high to low concentration". This action is called diffusion. It does not require energy.
2) the second part is "thru the plasma membrane". Here is where it can get confusing and it is important to recognize WHAT is moving. Is it the water or the particles? The particles! Therefore, the answer CANNOT be osmosis (the movement of water from high concentrations of water to low concentrations of water). Here we are being told the particle is moving from a high concentration of particles to a lower concentration of particles. Therefore the possible answer is called "passive diffusion". This is when particles can pass through open channels through a membrane. It is passive because no energy is used.
Movement thru a plasma membrane: http://classes.midlandstech.edu/carterp/courses/bio225/chap04/figure_04_17_labeled.jpg
*I forgot to mention passive diffusion can be further broken down into facilitated diffusion and simple diffusion. Both require no energy and are therefore "passive". I wrote the definition of "passive diffusion" incorrectly above. It is not JUST passage through a channel (facilitated diffusion)*
I would still just say osmosis, but it depends on your definition. I've adapted the following definition:
"osmosis the spontaneous passage of a pure solvent into a solution separated from it by a semipermeable membrane".
And I define semipermeable membrane as a membrane permeable to the solvent but not to the solute.
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It can either be osmosis or diffusion, but it depends on the molecule you have really (water vs. non-water). Since osmosis is a type of diffusion its safe just going with diffusion, since you don't know the particle.