Someone says: 'Some of the birds in this cage are green.' It turns out that this statement is FALSE. Which of the following must then be true?

'Some are green' claims that at least one green bird is in the cage. For that claim to be false, there must be no green bird at all, so the opposite of 'some are' is 'none is'. The traps: 'all are green' is the opposite of 'all', not of 'some', and it would actually make the original claim true rather than false; 'some are not green' can hold happily alongside some green ones, so it does not contradict the claim; and 'exactly one is green' again makes the original claim true.