The obvious answer (z) is not correct. There is a missing letter in this sequence--the letter Q. So ask yourself 'Where have I seen an alphabet without the letter Q?'

The answer is: these are the letters which appear above the numbers on the number pad or dial of a telephone. The sequence also does not contain the letter Z. It does, however, contain the following letters after the alphabet (above the number 0):

OPER

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Note of apology for international visitors: a couple solvers from outside the USA pointed out that in their countries, the telephone arrangement was very different. Anyone whose answer involved a telephone dial received partial credit for their solution.