In which of the following groups of organisms does excretion take place only by simple diffusion through the body surface?

Single-celled organisms like Paramecium, Amoeba and Euglena have no special excretory organs. Their waste simply diffuses out through the body surface.

Grasshoppers, snakes and toads have special organs such as Malpighian tubules or kidneys, so the other groups do not fit.