The biggest threats to christmas island crabs during their annual migration are a. dogs c. automobiles b. cats d. fences

The correct answer is c.  automobiles

Explanation:

Christmas Island red crabs are one of the 14 species of crab that lives on Christmas Island. The population of these crabs is decreased now but at one time it was more than 45 million. Introduction of an invasive ant species called yellow crazy ants decreased the population of there red crabs.

These crabs undergo massive annual migration towards the ocean to lay their eggs in the ocean. In their migration they need to cross highways and roads which poses the biggest threat to their life because the vehicles run over them and kills thousands of crabs.

So now government block those road which comes in the path of the migration of these crabs to protect these crabs. Therefore the right answer is c. automobiles.