The question is stated wrong.
It should be: "What is the next letter in the series O T T F F ?", and the answer is " S ".
These are the initial letters of the words: One Two Three Four Five, followed by Six.
what is the next letter e h l q
X
S
Q
c
c is next
c is next
c is next
y
E
S
The next (and last) letter is C. Just as a countdown counts backwards, this series is "countdown" spelled backwards.