1. The problem is to find the next term in the series: CR, LB, TM, SI, ? 2. Break down each term into two letters and analyze positions in the alphabet: C=3, R=18; L=12, B=2; T=20, M=13; S=19, I=9. 3. Analyze first letters: C(3), L(12), T(20), S(19). Differences: 12-3=9, 20-12=8, 19-20=-1. 4. Look for a pattern in first letters: +9, +8, -1; pattern unclear so check second letters. 5. Second letters: R(18), B(2), M(13), I(9). Differences: 2-18=-16 (or +10 if cycle through alphabet considered), 13-2=11, 9-13=-4. 6. Try checking pairs as 2-letter codes or checking alternations: first letters jumping down from T to S suggests letters near middle alphabet. 7. Another approach: letters are pairs with some pattern - e.g., C and R are symmetric around middle? Or positions of letters in word? 8. Given options, check each pair's numbers: PK (P=16, K=11), DK (D=4, K=11), DG (4,7), NX (14,24). 9. Assume last term SI (19,9), next first letter could be P(16) from PK (16,11), matching an increasing-decreasing letter pattern. 10. Selecting PK fits best pattern and options. Final Answer: PK