Most of those options are not really feasible.

It should ignore Iran.
A belligerent, nuclear, Iran could, and has, easily send oil prices soaring. Far too detrimental to the economy, and far too dangerous to stability. Especially with the US parked firmly in Iraq for the immidiate future, ignoring Iran's nuclear ambitions is not an option.

It should invade Iran.
With what? US groundtroops are already deployed far too thin. To control Iraq the US has already had to deploy far too many reservists (violating their contracts). Furthermore, the Iranian military is no pushover. It has some pretty advanced equipment and is large. Any war would be a high casualty affair, something that an already war-weary US public would not swallow.

It should bomb Iran's nuclear facilities but not invade Iran.
Sure sounds nice on paper, but it's not quite that easy. Iran's nuclear program is spread around the country, and cannot be stopped with a swift strike like Iraq's in the 80's. Even if it were hit, it would only be a question of time before Iran regains lost ground.
Like North Korea, Iran has a lever over the US; namely Iraq. Iran could put great pressure on the Shiite majority and increase the amounts of civil unrest. Enough to topple the government? Probably not, but surely enough to make governing a very difficult task. And then there's the whole Arab unrest, and disruption of supplies of Iranian oil factor...

Regardless, I'm pretty sure Israel will try it, with US connivance.

It should work through the U.N. to attempt to prevent Iran from having nuclear weapons.
Heh. Yeah, I'm sure Iran will listen to Kofi. After the complete failure of the UN to do anything about the US invasion of Iraq, the organization has been so completely discredited that it would be a massive political loss for Iran to comply with the UN.
Diplomatic/Economic levers could work, but as long as Russia and China are not willing to use them, any such action would be pointless.

It should allow Iran to have nuclear weapons and use diplomacy to prevent conflict.
With the dismal possibility of any other option succeeded, it looks like we are headed towards this. The question will soon be how much of Iran's leaders (no way I'm gonna try and spell that name) anti-Israel rhetoric is populist and how much do realpolitik concerns matter. I still think it is unlikely that even a nuclear Iran is so apocalypse eager as it sounds.
For the first time ever, I'm happy not be living in Israel at this time.