Dr. Skolnick believes that the rise in food allergies may be explained in part by (us having become too hygienic of a society)
Correct answer: (the fact that we have become too hygienic a society)
Why is the original incorrect?
I don’t have a neat-and-tidy rule to cite here—there might be one but I don’t know its name.
However, I can tell you that it doesn’t work to say, “…us having become…” It would be OK to say, “…our having become…,” but that’s not an answer choice.