Let's analyze the relationship between the letter groups:
- CAT : DDY
Look at the positions of letters in the alphabet:
C (3), A (1), T (20)
D (4), D (4), Y (25)
Difference between each corresponding letter:
3 → 4 (+1), 1 → 4 (+3), 20 → 25 (+5)
So, the increments are +1, +3, +5 respectively.
Apply the same to BIG:
B (2), I (9), G (7)
Adding +1, +3, +5 respectively:
2 + 1 = 3 (C), 9 + 3 = 12 (L), 7 + 5 = 12 (L)
Therefore, the answer is CLL.