Answer is "O" as each letter is first letter of the consecutive number.
its O
one=O two=T three=T four=F ............................ .............
E
as 1st and 3rd row are same & 2nd and 4th row are same. So, E will replace ?
Which Letter Replaces the ? mark Read Ans is - E
Which number replaces the question mark - 12, 9, 7, 3, 4, 6, ??