Which of them is the limiting reagent on the production of terrorist attacks? Not the means; guns, explosives, and other weapons aren't difficult to come by or create. Not the opportunity; we do not armor all possible terrorist targets nor track every potential terrorist. Rather, the limiting reagent on the production of terrorist attacks is motive — people's interest in doing them.

That depends on your location. There is no shortage of would-be suicide bombers in the Palestinian territories (or so I have been led to believe). There is, however, a shortage of explosives and access to targets within Israel.