Final answer:
The correct option is d) Overpopulation in their previous habitat is forcing migration.
Step-by-step explanation:
According to the provided information, one theory about the recent migration of crayfish is that climate change is causing shifts in their habitat. The incidents of collapsed signal crayfish populations in Nordic countries indicate that the crayfish are losing their advantage, which suggests that pollution in their previous habitat has made it uninhabitable (option b).
The information also suggests that crayfish plague epidemics and subsequent infections, as well as competition due to translocations, can lower the reproduction potential of crayfish stocks. This indicates that predators driving them to seek new areas for protection (option c) and overpopulation forcing migration (option d) are possible theories explaining the crayfish migration.