Select the option that is related to the fifth letter-cluster in the same way as the second letter-cluster is related to the first letter-cluster and the fourth letter-cluster is related to the third letter-cluster.
ECHO ∶ PIDF ∶∶ EARS ∶ TSBF ∶∶ HEAR ∶ ?
1
IFBS
2
SCFI
3
SBFI
4
SBGI