(1) No such object has struck the Earth-Moon system for the last 3.8+ Ga, so the odds are low

(2) No way that we could deflect something that massive

(3) but at least we would have many years (to decades) notice of the arrival of such an object. The current search for KBO's that has demoted Pluto to be a "not-quite-planet" is showing no threats to Earth on a 50-year baseline.

(4) 50 years warning is time to develop space transportation, build colonies on the Moon, Mars, Asteroids, and generally diversify...

of course not everyone will survive. In fact, most will perish. It's called "evolutionary pressure"...