Simple diffusion is the correct answer. All the others depend on transport carriers (specialized protein molecules) which act to move the transported molecule into the cell. There are only so many of those carrier proteins in the cell membrane, so when they are all "tied up," then the system is saturated. If you do a graph of uptake into the cell, you see a hyperbolic curve, where at low concentrations of substrate, the uptake is fast and linear. But as you go higher, the curve flattens out. That flattened part of the curve is where there are no more carriers available to carry in more stuff--i.e. it's now "saturated." If you have, say 100 carrier molecules, then the maximum number of things you can transport is 100--at 100, the system is saturated. Even if you increase the number of things to 200 (i.e. doubling the concentration gradient), you still only have 100 carriers, so you can't go any faster than 100. Only way to increase it is to produce more carrier molecules and put them into the membrane.
A graph of simple diffusion would be a straight line that keeps going steadily up--linear uptake which doesn't flatten out--i.e. doesn't saturate. This form of transport doesn't saturate because it only depends on a) the difference in concentration of the moving molecule between the outside and inside of the cell, and b) the available surface area across which the molecule must diffuse--IOW, it's movement is directly across the membrane, and doesn't depend on a finite number of "helper" carriers to get across.