Answer:
C. birds moving back and forth from the mainland to islands and back again each week
Step-by-step explanation:
In each of these examples, a process called geographic isolation takes place. This is where a physical barrier separates a population, forcing them to adapt to different environments and causing speciation to take place over time.
In A. a major river changes course and splits a valley into two parts where a single squirrel species lived. this species was a particularly poor swimmer, and couldn't get together with the squirrels on the other side of the river, geographic isolation takes place, as a river splits the population in two, taking away interaction.
In B. a volcano erupts and lava flows into a valley, changing the light colored sand landscape, where light colored lizards live, to a dark black, geographic isolation also takes place. One part of the valley is light colored, while the other part is dark black. The lizards living in the light colored area will most likely stay the same. In the dark black area, however, the light colored lizards are forced to adapt to the new color of the land, where predators can more easily spot them.
In C. birds moving back and forth from the mainland to islands and back again each week, geographic isolation does NOT take place. The birds are still able to interact with each other, as they can travel from the islands to the mainland without anything preventing them.
In D. some insects are blown many miles offshore to a group of islands and can't return to the mainland, geographic isolation does takes place. The insects are blown away, and the sea and the distance away acts as a barrier, preventing the insects from interacting and causing them to become two separate species.