Answer:
The correct answer is option A which is, "Disease and predators".
Step-by-step explanation:
The decrease in beetles population is the result of disease and predators for the fog beetles.
In the given option, like in A the habitat loss can be one reason but the immigration is not the reason. Because immigration means the moving into that area from the foreign area. Immigration leads to the increase in population size not decrease.
And disease is some abnormality related to beetles which leads to their death and ultimately decrease in population size.
And predators are some other insects and birds which use them as their food and is limiting factor.