The options for questions here are a bit skewed since they don't reflect varying combinations of probability and effect. Really there ought to be two polls here - one is how likely is another terrorist attack, the second would be how damaging to the economy.

I believe the probability of another major terrorist attack within the next two years is so high that it reaches certainty. It's quite possible (again reaching certainty) that there have been such attacks attempted and the parties making the attempt are now no longer with us.

Its possible that a relatively minor attack could have a major financial impact; also that an apparently massive and damaging assault could have much less. Just too many variables to judge.

But, there will be another attack; my guess is probably biological. Get your smallpox shots up to date.