A series of two letter terms is given with one term missing. Choose the correct alternative from the given options that will complete the series RP, DM, SQ, CL, TR, ?

Introduction / Context:
This alphabet test problem contains a series of two letter combinations where the first and second letters follow two interleaved patterns. Recognising that alternate terms may form separate sub series is the key to solving this type of question. The learner has to correctly separate and analyse the odd and even positioned terms.

Given Data / Assumptions:

• The series is RP, DM, SQ, CL, TR, ?
• Each term consists of exactly two uppercase letters.
• Positions in alphabet: A = 1 through Z = 26.
• The pattern may involve two interlocking sequences based on position parity.

Concept / Approach:
We first check whether the first, third, and fifth terms form one sub series and the second and fourth terms form another sub series. Often, alternating terms follow one rule for the first letters and another for the second letters. By analysing these sub sequences, we can determine what the missing sixth term should be.

Step-by-Step Solution:
Step 1: List the terms with indices: 1st RP, 2nd DM, 3rd SQ, 4th CL, 5th TR, 6th ? Step 2: Consider the odd indexed terms: RP, SQ, TR. Step 3: First letters of odd terms are R, S, T. Their positions are 18, 19, 20, which shows an increment of plus 1 each time. Second letters are P, Q, R with positions 16, 17, 18, also plus 1 each time. Step 4: So the odd indexed sub series is a simple plus 1 progression for both letters, and the next odd term after TR would be US. However, that would be the 7th term, not the 6th, so we keep this observation in mind. Step 5: Now look at the even indexed terms: DM, CL, and the missing 6th term. Step 6: For DM, D is 4 and M is 13. For CL, C is 3 and L is 12. Both first and second letters decrease by 1 when moving from DM to CL. Step 7: Applying the same minus 1 step again, we go from C (3) to B (2) and from L (12) to K (11). So the sixth term, which is the third even term, must be BK.

Verification / Alternative check:
We can verify that RP, SQ, TR indeed form a clean ascending sequence where each letter advances by one position, while DM, CL, BK form a descending sequence where each letter moves back by one position. These two patterns are neatly interwoven to produce the full series. No other option for the sixth term preserves this structure.

Why Other Options Are Wrong:
AJ and US do not fit the minus 1 pattern for the even indexed sub series. US actually would be the correct continuation of the odd sub series after TR, but it occupies the wrong position in the overall sequence. DM repeats the second term and disrupts the distinct progression. Therefore, only BK fits logically as the sixth term.

Common Pitfalls:
Students sometimes try to find one single rule for all terms and miss the fact that there are two interleaved sequences. Others may incorrectly assume that the next term must continue the pattern seen in the immediate previous term instead of examining index based grouping. Recognising and separating alternating sub series is an essential skill for such problems.

Final Answer:
The correct pair that completes the series is BK.