Step 1: Concept
Letter series progression.
Step 2: Analysis
Convert letters to their numerical positions:
C=3, F=6, I=9, L=12, O=15.
Step 3: Calculation
The pattern is an increase of 3 each time.
The next number is $15 + 3 = 18$.
Step 4: Conclusion
The 18th letter of the alphabet is R.
Final Answer: (4)