The shrinking habitat is a separate possible explanation for the dwindling numbers of amphibians, unrelated to the ozone layer. We can infer this from the passage, which notes that the loss of ozone is particularly bad for amphibians, because they don’t have hair, hide or feathers – it affects them directly, not through their habitats. Loss of habitat might be something like deforestation, global warming, etc.
If D *offered* another explanation, it would be a good answer; answer choices that provide alternate expiation weaken the argument. But D actually *eliminates* the alternative explanation (“…has NOT become smaller…” (emphasis added)). Answer choices that ELIMINATE possible alternate explanations strengthen arguments, because whatever the percentage chance that the alternate explanation was correct gets “absorbed” by the other explanations, including the argument’s.
For instance, let’s say ozone layer and loss of habitat are about equally likely, when the argument is made, with a 20% chance that it’s someing else. So after we read the passage, the odds are::
40% ozone loss
40% loss of habitat
20% something else.
Now we add (D) into the mix, and that 40% loss of habitat is eliminated. Where does it go? It goes to the remaining possibilities. So now, maybe it’s:
60% ozone loss
40% something else.
The argument has gotten stronger, because the argument’s conclusion is more likely to be true,
Strengthen and weaken questions can operate in different ways, but the most common way pertains to alternate explanations – the right answer in weaken questions (often) provides them, and the right answer in strengthen questions (often) eliminates them.
Hope this helps.