In the following alphabet series, various terms are given with one term missing as shown by (?). Choose the missing term from the given alternatives: BF, CH, ?, HO, LT.

Introduction / Context:
This question involves an alphabet series in which each term consists of two letters. You are given several terms with a missing pair in the middle. The task is to determine the missing pair by analysing the pattern in how both the first letters and the second letters move through the alphabet. Such questions assess your skill in mapping letters to positions and tracking regular jumps in value across several steps.

Given Data / Assumptions:

• Given series of letter pairs: BF, CH, ?, HO, LT.
• Exactly one two letter term is missing in the third position.
• The English alphabet has positions A = 1, B = 2, ..., Z = 26.
• The pattern is expected to apply separately to the first letters and the second letters of each pair.

Concept / Approach:
For such series, it is best to separate the pattern for the first letters (B, C, ?, H, L) and for the second letters (F, H, ?, O, T). Convert letters into their numeric positions and examine the differences. Often, each subsequence (first letters and second letters) forms its own arithmetic progression, possibly with a different step size. The missing pair must respect both of these progressions at the same time.

Step-by-Step Solution:
Step 1: Consider the first letters: B, C, ?, H, L. Their positions are: B = 2, C = 3, H = 8, L = 12. Step 2: Look for a pattern. A natural sequence with these values is 2, 3, 5, 8, 12 where the missing first letter is at position 5. Step 3: Position 5 corresponds to the letter E. So the first letter of the missing pair is E. Step 4: Now consider the second letters: F, H, ?, O, T. Their positions are: F = 6, H = 8, O = 15, T = 20. Step 5: Differences: 8 - 6 = 2, 15 - 11 = 4, 20 - 15 = 5 suggests we want a progression 6, 8, 11, 15, 20 with steps +2, +3, +4, +5. Step 6: The missing second letter should therefore be position 11, which is K. Step 7: Combining both parts, the missing pair must be EK.

Verification / Alternative check:
Write the completed sequence using EK: BF, CH, EK, HO, LT. First letters in positions: B(2), C(3), E(5), H(8), L(12) follow increments of +1, +2, +3, +4. Second letters F(6), H(8), K(11), O(15), T(20) follow increments of +2, +3, +4, +5. This double arithmetic pattern is clean, simple, and consistent, strongly confirming EK as the correct missing term.

Why Other Options Are Wrong:
DN, EL, and EM do not satisfy both progressions at the same time. If we insert any of them, then either the first letters or the second letters will fail to follow a regular sequence of increasing differences. For example, using EL produces second letters F, H, L, O, T which break the pattern of differences +2, +3, +4, +5. Hence these options are inconsistent with the underlying structure of the series.

Common Pitfalls:
Some learners focus on letter shapes or try to guess based on partial observations without converting letters to positions. This often hides the simple arithmetic pattern. Another mistake is to treat the pair as a whole and search for complex transformations instead of separating the first and second letters into two clean subsequences. Always analyse each column of letters independently in such problems.

Final Answer:
The missing term that completes the alphabet series is EK.