Option Zero was the very last weapon of the UEF, only to be used if the UEF were about to be defeated. It involves a high amount of nuclear weapons being launched simultaneous all over Earth and then impacting, destroying all life on Earth. President Riley is terrified of using Option Zero, personally stating it in a transmission to you. He does however use it if you lose the last mission in the Aeon Campaign, which is luckily not canon.