Select the pair that follows the same pattern as that followed by the two…

Concept: a letter-pair pattern is fixed by two independent things — the internal shape of each three-letter group, meaning the alphabetical gaps between its own letters, and the constant shift that carries the first group onto the second. A candidate pair belongs to the same family only when it reproduces both of them.

1. Write the alphabet positions of CAC: C = 3, A = 1, C = 3. The outer letters are identical and the middle letter sits 2 places earlier, so the shape is X, X−2, X.

2. FDF gives F = 6, D = 4, F = 6 — the same shape X, X−2, X.

3. Now measure the shift from CAC to FDF: C (3) → F (6), A (1) → D (4), C (3) → F (6). Every position advances by exactly +3.

4. Test the rule on the second example: OMO = 15, 13, 15 and RPR = 18, 16, 18 are both X, X−2, X, and O (15) → R (18), M (13) → P (16), O (15) → R (18) is again a uniform +3.

5. So the governing rule has two parts: each group must read X, X−2, X, and the second group must be the first group shifted forward by 3.

Only 20, 18, 20 → 23, 21, 23 keeps the X, X−2, X shape in both groups and moves every position by the same +3. TRT-WUW is therefore the pair that follows the pattern of CAC-FDF and OMO-RPR.