Solution: The correct answer is CCF. The pattern is: RRB → TTD (each letter moves forward by 4 positions: R→T, R→T, B→D). Applying the same to AAD: A+4=E, A+4=E, D+4=H → EEH, but not in options. Alternatively, it might be a code: R=18, T=20 (+2), R=18, T=20 (+2), B=2, D=4 (+2). For AAD: A=1→C=3 (+2), A=1→C=3 (+2), D=4→F=6 (+2). So AAD → CCF.