A letter series is given as A, C, F, H, K, ? Based on the English alphabet, which letter should come next to continue the pattern correctly?

Introduction / Context:
This series question asks you to identify the rule that generates a sequence of single letters: A, C, F, H, K, and a missing term. Such problems are part of alphabet test in aptitude exams and they train you to think in terms of numerical positions of letters and alternating patterns of jumps rather than simply looking at the letters as sounds or words.

Given Data / Assumptions:

• Series: A, C, F, H, K, ? • Options: M, N, L, O • Alphabet positions: A 1, B 2, C 3, D 4, E 5, F 6, G 7, H 8, I 9, J 10, K 11, L 12, M 13, N 14, O 15. • We assume there is a consistent pattern of steps between successive letters.

Concept / Approach:
Many letter series use a repeating pattern of different step sizes. A common technique is to convert each letter to its numeric position and calculate differences between neighbouring terms. If these differences themselves follow a pattern, such as alternating or gradually increasing values, we can extend them to find the next letter in the sequence. Here we particularly look for a repeating pattern of two step sizes.

Step-by-Step Solution:
Step 1: Convert letters to positions: A 1, C 3, F 6, H 8, K 11. Step 2: Find differences between successive terms: C minus A is 3 − 1 = 2, F minus C is 6 − 3 = 3, H minus F is 8 − 6 = 2, K minus H is 11 − 8 = 3. Step 3: The pattern of differences is therefore +2, +3, +2, +3 and so on, alternately. Step 4: Following this alternating pattern, the next difference after +3 should be +2 again. Step 5: Add 2 to the last position: 11 + 2 = 13. Position 13 in the alphabet is M. Step 6: So the missing term must be the letter M.

Verification / Alternative check:
We can verify by writing the extended series with positions: 1 (A), 3 (C), 6 (F), 8 (H), 11 (K), 13 (M). The differences remain +2, +3, +2, +3, +2, which perfectly preserves the alternating jump pattern. None of the other options N, L, or O maintain this alternation when inserted at the end, because they would give differences of +3, +1, or +4, which breaks the observed regularity. This confirmation shows that M is the only valid continuation.

Why Other Options Are Wrong:
If we choose N (14), the last difference becomes 14 − 11 = 3, so the pattern of differences becomes +2, +3, +2, +3, +3, which breaks the alternation. If we choose L (12), the difference from K is only 1, which does not match either +2 or +3. If we choose O (15), the last jump is 4, which again is inconsistent with the established +2, +3 pattern.

Common Pitfalls:
Students sometimes see a partial pattern such as increasing or random steps and guess quickly without fully checking the differences. Another error is to miscalculate positions of letters like H, K, or M, which lie around the middle of the alphabet. Writing down the positions and calculating the differences systematically helps prevent these mistakes and makes the alternating step pattern very clear.

Final Answer:
The correct letter to continue the series A, C, F, H, K is M.