I would say that <u>a deadly water-borne microorganism in a city's water source</u> would most quickly devastate a human population, merely because a huge number of people uses this water for some kind of a purpose.
Head injury, as your head is your skull, damaging your skull is very serious as your brain is located there and damaging your brain could probably kill or limit some abilities
The answer is A
Hope this helps! :3