The comma goes before the "but" I don't know how the wrong answers get rated so highly. Just because someone explains why they're "right" doesn't mean that they are.
Conservationists are considering urban runoff and they are tracking the population growth, but the magnitude of the situation has not been determined.
Answer: "Conservationists are considering urban runoff, and they are tracking population growth, but the magnitude of the situation has not been determined."
