The answer should be A, particles move from an area that is more concentrated to an area that is less concentrated.
The process of diffusion happens naturally, no extra energy is needed for this process. Think of a cup of water. If you put a drop of blue ink to the cup of water, you will see that at first the ink stays in a drop, then slowly spread out to the other parts of water until the whole cup of water is blue. This is the process of diffusion. The ink particles moves from a area of higher concentrated (the drop) to the area of lower concentration (the rest of the water). The net movement of diffusion happens until equilibrium, where both sides of the solution has a same concentration of solute (ie. The whole cup of water turns blue). At this time, the particles still move freely, but they don't have a net movement (the particles moving to the left side equals to the one moving to the right).
Note that diffusion applies to any fluid (liquid/gas), and the net movement must be down a concentration gradient.
