Select the option that best completes the letter-and-number analogy: C3 : F6 :: I9 : ?

In every term the digit gives the place of the letter — C is 3rd, F is 6th, I is 9th — and each term moves 3 places on. Three past I (9th) is L (12th), so the digit must be 12, giving L12. 'K11' keeps the letter and its digit in step but moves only 2 places, while 'L11' and 'M12' break the rule that the digit must equal the letter's place.