The best option is not screwing up our planet. If asteroid strikes is the concern, then funding a planetwide network of residents living in underground bunkers with supplies to last decades is probably cheaper and more effective than going to Mars.
That isn't even close to the best option. That's like saying: "don't make backups, just don't screw up."
edit: to be more specific, "make makeups, but also try not to screw up"
That does not even belong in the same category. Data backups in computer systems are cheap, and people/organizations not doing them even cheaper!
On the other hand, doctors don't make backups of the gravely ill[1] before a dangerous surgery. That is beyond our current capabilities, so "do not screw up" is as good as it gets.
Yet another example, civil engineers do not make backups of skyscrapers[2] before doing maintenance work, even major maintenance work. While technically feasible, the economic cost would be prohibitive.
My gut feeling is that a "backup planet" would fall somewhere in between cases [1] and [2].
It's very hard to imagine what we could do to Earth so that it becomes even less hospitable than mars.
If "lifeboat for species" is the goal, then going to Mars is a solution in that direction but not a particularly good one - building an underground colony in Antarctica or a self-sustainable isolated underwater colony would achieve the goal better, be reachable quicker, and at a lower cost. However, 'lifeboat for species' right now is not an explicit end goal for anyone who would be capable to fund that.
We're not looking for a lifeboat, humanity is simply looking for another planet to screw up.