The correct answer is - C. Humans.
The stickleback in Loburg Lake, Alaska, is not a native species of the lake. It is thought that this fish was introduced to this lake somewhere between 1984 and 1989. The reason for this kind of suggestion is that the stickleback in this lake had full armor like the stickleback in the seas, which is not something found among the lake populations of this species. Also, the full armor had gradually been reducing with each new generation, and in the present, the stickleback in this lake looks totally the same as the other lake populations.
The stickleback did not had any predators in Loburg Lake, thus the losing of the full armor as it was not needed, instead the stickleback was the predator. But even though it didn't had a predator in the water, the humans became its predator through the recreational fishing that takes place on this lake.