A-higher birth rates and death rates. No, this is not a problem but a matter of the life cycle.
b-the breakdown of infrastructure. Infrastructure will most likely reach its peak during overpopulation as you need stuff to keep people alive.
c-closing of locally owned factories. I do not think so, most likely more factories will open due to more supplies needed.
d-the spread of disease worldwide. This makes the most sense, it is a problem with overpopulation.
Therefore, I believe the answer is D.
I hope this helps! :)
