A massive multinational invasion would be another option - I don't even think the US public would support such an endeavor that would dwarf the Iraq invasion in terms of costs in money and casualties - let alone any other Western nation. And apart from the extremely unlikely public support I also have some doubts that the massive loss of lives, the economic disaster and the resulting instability of the region would be justified by what could be gained by it (although Iran is currently one of the last nations I would like to see having access to nuclear weapons - objectively we already have
a nation that could easily turn into a similar extremist state during the next 5 years and that already has
nuclear weapons: Pakistan).
I won't even go into pre-emptive nuclear strikes as I think that most sane people will not consider this to be an actual option - once this becomes a viable option, we are entering a new era that I would rather not witness during my lifetime.
The bottomline seems to be that, in the end, all that the world seemingly can realistically do if Iran really
is going after nuclear weapon technology, is to watch and hope that responsibility and the realization that nukes are a weapons that only gives you safety as long as you do not use it go hand in hand with the gain of power.
Do I like that idea? No! (and this development could hardly come at a worse time, with a seemingly mad demagogue being in a powerful position in Iran).
But unfortunately we will probably have to get used to the fact that Iran will not be the last nation that gains access to nukes (and which we would rather like not to have access).
To use a quote from Babylon 5
"The avalanche has already started - it is to late for the pebbles to vote"