Choose the letter that correctly completes the series: Y, B, T, G, O, ?

Introduction / Context:
This problem involves an alphabetical series where you must identify the pattern and choose the correct missing letter. Letter series questions check your ability to map letters to positions in the English alphabet and to track regular jumps or alternating patterns between them.

Given Data / Assumptions:

• Series: Y, B, T, G, O, ?
• Exactly one letter is missing at the end.
• The English alphabet has 26 letters from A to Z.

Concept / Approach:
For letter series, it is often useful to separate the letters into two interleaved sub series and look at positional differences. Convert each letter to its numeric position (A = 1, B = 2, ..., Z = 26). Then examine patterns like constant jumps, descending steps, or other simple progressions. Because the letters seem to alternate between larger and smaller values, we suspect two independent sequences.

Step-by-Step Solution:
Step 1: Write positions: Y = 25, B = 2, T = 20, G = 7, O = 15. Step 2: Separate into odd and even positions. Odd position letters: 1st Y (25), 3rd T (20), 5th O (15). Even position letters: 2nd B (2), 4th G (7), 6th unknown. Step 3: Look at odd positions: 25, 20, 15. Each step decreases by 5, so the next odd term after 15 would be 10, which corresponds to J. That would be the 7th term, not the missing 6th term, so we just note this pattern for later. Step 4: Look at even positions: 2, 7, ?. Here the step from 2 to 7 is +5. To continue a simple pattern, we again add 5: 7 + 5 = 12. Step 5: Position 12 in the alphabet is the letter L. Therefore the missing 6th term must be L.

Verification / Alternative check:
Write the two interleaved sequences explicitly: odd positions Y, T, O, J (25, 20, 15, 10) each step -5; even positions B, G, L (2, 7, 12) each step +5. This yields the full sequence Y, B, T, G, O, L, J, which follows two clean progressions. The given part of the sequence is consistent with this rule, which confirms L as the only possible missing letter.

Why Other Options Are Wrong:
M, N, and K correspond to positions 13, 14, and 11. None of these continue the +5 pattern from 2 to 7. If we try 11 instead of 12, the steps become +5 and +4, which is irregular. Therefore these options cannot maintain a simple arithmetic sequence in the even positions and must be rejected in favour of L (12).

Common Pitfalls:
Many students attempt to work directly on the whole list of letters and fail to see that two sequences are interwoven. Always try splitting alternate terms for alphabet questions when a direct pattern is not obvious. Also remember to use numeric positions, because arithmetic reasoning is much easier with numbers than with letters alone.

Final Answer:
The letter that correctly completes the series is L.