Unfortunately you do not tell us which trait is dominant so we need to assume that dark mice light mice and medium colored are all possible.
The answer is 1...........
As the pythons are an invasive species, they would have no natural predator, That eliminates C & D. They don’t eat fish so B is out. Pythons could eat alligator eggs and besides the bounty placed on the pythons, the only thing biologist should do is protect the alligator eggs.
Caleb’s immune system will have the experience of getting rid of this antigen and know exactly what to do to overcome it efficiently.