Solution

Gause grew populations of Paramecium aurelia and the larger P. caudatum, in controlled conditions. When these two populations were grown in separate test tubes, the population of each species quickly increased to the carrying capacity. When grown together, determined by available conditions, one species thrived over other. It led to the principle of competitive exclusion that two competing species can not coexist and in time one or the other would eventually out-compete.
Therefore, the correct answer is option D.