What disaster has the potential to kill the most people in Canada?

Justin Bieber...

A 10 mile diameter meteor.

I am sure they are asking about "potential" in terms of also likely, so it would have to be earthquakes. But truly, a major meteorite impact could take out the entire earth, so "potentially" that would be the most deadly. Given that the frequency of major impacts is on the order of millions to hundreds of millions of years, though, I doubt anyone should worry about it. A major earthquake out west, though, is very probable within our lifetime, and could cause tens to hundreds of thousands of deaths (although it probably wouldn't/won't cause that many; hundreds would not be surprising, which is way more than any flood-landslide-hurricane will cause).